How can I get the currently playing clip?

When you say about to start, do you mean 'is starting this frame' or will start within a given time period?

If it's this frame, you can override OnBehaviourPlay in your clips PlayableBehaviour (LightControlBehaviour) set a variable to true. Then in the ProcessFrame above (i.e the PlayableBehaviour for the track) you can check the value on each input, make a change and then set it to false. something like:

Code (CSharp):

1. class LightControlBehaviour : PlayableBehaviour
2. {
3.     public bool becameActiveThisFrame;
4.     public override void OnBehaviourPlay(Playable playable)
5.     {
6.         becameActiveThisFrame = true;
7.     }
8. }
9.  
10. class LightControlMixerBehaviour : PlayableBehaviour
11. {
12.     public override void ProcessFrame(Playable playable, FrameData info, object playerData)
13.     {
14.         int inputCount = playable.GetInputCount(); //get number of clips on track
15.         for (int i = 0; i < inputCount; i++)
16.         {
17.             ScriptPlayable<LightControlBehaviour> inputPlayable = (ScriptPlayable<LightControlBehaviour>)playable.GetInput(i);
18.             LightControlBehaviour input = inputPlayable.GetBehaviour();
19.             if (input.becameActiveThisFrame)
20.             {
21.                 input.SetSomeValueHere();
22.                 input.becameActiveThisFrame = false;
23.             }
24.         }
25.     }
26. }

Otherwise if you want to set a value in advance of it becoming active, it's a bit trickier, but one formula that works is

1. Add a startTime property or non-serialized field to your PlayableAsset and PlayableBehaviour for the clip. In CreatePlayable, make sure to copy it from the PlayableAsset to the PlayableBehaviour.
2. In your tracks CreateTrackMixer method, set the start time of the playable asset. CreateTrackMixer gets called before CreatePlayable.

Code (CSharp):

1.       public override Playable CreateTrackMixer(PlayableGraph graph, GameObject go, int inputCount)
2.         {
3.             foreach (var clip in GetClips())
4.             {
5.                 var asset = clip.asset as MyPlayableAsset;
6.                 if (asset != null)
7.                 {
8.                     asset.startTime = clip.start;
9.                 }
10.             }
11.  
12.             return ScriptPlayable<MyMixerPlayableBehaviour>.Create(graph, inputCount);
13.         }
14.  

3. In your mixer PlayableBehaviour you can use the timeline time to see if you are within a given range of the clip.

Code (CSharp):

1. public override void PrepareFrame(Playable playable, FrameData info)
2. {
3.     var timelineTime = playable.GetGraph().GetRootPlayable(0).GetTime();
4.     for (int i = 0; i < playable.GetInputCount(); i++)
5.     {
6.         ScriptPlayable<MyPlayableBehaviour> scriptPlayable = (ScriptPlayable<MyPlayableBehaviour>) playable.GetInput(i);
7.         MyPlayableBehaviour myPlayableBehaviour = scriptPlayable.GetBehaviour();
8.         double clipStart = myPlayableBehaviour.startTime;
9.  
10.         if (timelineTime > clipStart - 0.5f && timelineTime <= clipStart)
11.             myPlayableBehaviour.DoSomething();
12.     }
13. }
14.