Make use of a key exercise? (5-2)
I believe the answer is:
press-up
'exercise?' is the definition.
(I know that press-up is a type of arm exercise)
'make use of a key' is the wordplay.
'make' becomes 'press' (pressing is a kind of making).
'use of a key' becomes 'up' (I can't justify this - if you can you should give a lot more credence to this answer).
'press'+'up'='PRESS-UP'
(Other definitions for press-up that I've seen before include "Form of arm exercise" , "Arm exercise while lying face down" , "Face-to-the-floor exercise" , "Keep-fit exercise" , "Gymnastic floor exercise" .)