newbie sidechaining question

[samu23]

Hello,
Can anyone tell me if there's a way to set up a sidechain with say a bass drum playing 4 beat bar and a synth that's getting effected by the sidechain, but only have the synth playing, or do you have to have both instruments to play for the effect to work, I'm using ableton 9, please help a newbie cheers

[samu23]

Now sorted please ignor