Strict, firm with fleet (4,3,4)
I believe the answer is:
hard and fast
'strict' is the definition.
('hard-and-fast' can be a synonym of 'strict')
'firm with fleet' is the wordplay.
'firm' becomes 'hard' ('hard' can be similar in meaning to 'firm').
'with' becomes 'and' (I've seen this before).
'fleet' becomes 'fast' ('fast' can be similar in meaning to 'fleet').
'hard'+'and'+'fast'='HARD-AND-FAST'
(Other definitions for hard and fast that I've seen before include "(Of rules) invariable" , "Rigidly applied" , "Not moving" , "(Of rules) unalterable" , "Not able to be changed" .)