Move suddenly forward in swim? Not at first (5)
I believe the answer is:
lunge
'move suddenly' is the definition.
(I've seen this before)
'swim? not at first' is the wordplay.
'swim?' becomes 'plunge' (plunge is a kind of swim).
'not at first' means to remove the first letter.
'plunge' with its first letter removed is 'LUNGE'.
'forward in' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lunge that I've seen before include "Trust forward as with sword" , "Thrust forward as in fencing" , "fast movement" , "Make a thrusting forward movement" , "Thrust, sudden pass" .)