I have a steady synth sound and a kick drum and have it set up to side-chain the kick as the side-chain source on the synth track’s vintage compressor. All good so far.
Now I want to be able to mute/mix the kick drum, but still hear the side-chain effect on the synth.
How is this done?
Change the send to pre fade and turn the Kick down.
Here’s another way –
Create a midi track (or audio) with a kik or other sound playing the pulse rhythm you like – be it four on the floor or other.
Go to the Music Track you want to process, the pad, for example, and place a compressor – “Not forgetting to activate the side chain button, Mr. Crusher,” said Picard 
– as an insert, or, just use the channel strip compressor and press the SC button. BTW, the envelop shaper on the strip also has SC.
Return to Rhythm Track and go to the Track Inspector and send the track to the Side Chain rather than to Stereo Out, or, you can do it with the Direct Routing Tab on the inspector (depending on version).
Play the tracks and adjust the side-chained Compressor on the Music Track so that track is doing what you want it to do – for example, keeping the kik or other sounds from being buried, or losing definition, or something else that you’re wanting to hear happening in the Dynamics of that individual track and, consequently, the entire piece…
I find I often have to use extreme settings at first to hear what’s happen and then I’m able to back off to something that’s making the music sound more, as they say, glued together, or, the dynamics are doing what I want them to do in terms of ducking or responding to the rhythm in an interesting way.
I’d like to hear any suggestions about use of Side-Chains for instruments and vocals. Be well. 