Left crank cracks when the left pedal is pressed
If it is an older 3-piece crank, and if you feel play on the left-side downstroke, then it could certainly be a loose cotter, but it could also be that the bearing cap needs to be tightened in a tad more.  I have an older 3-piece crank (w/ loose ballbearings) and I've had to do that before, especially after an overhaul when things tend to loosen up after a few rides.

Of course, there are many types of bottom brackets, as explained here:

https://en.wikipedia.org/wiki/Bottom_bracket