Bully someone doing sit-ups (6)
I believe the answer is:
abuser
'bully' is the definition.
(I've seen this before)
'someone doing sit-ups' is the wordplay.
I cannot quite understand how this works, but
'someone' could be 'user' (user is a kind of someone) and 'user' is present in the answer.
The remaining letters 'ab' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for abuser that I've seen before include "Person who over-indulges in, say, drugs or drink" , "He takes undue advantage" , "one who uses bad language" , "One mistreating another" , "One who reviles or a misuser of drugs" .)