A heavy blow during the midshipman's watch (4)
I believe the answer is:
swat
'a heavy blow' is the definition.
(I've seen this in another clue)
'during the midshipman's watch' is the wordplay.
'during' indicates the answer is hidden within the clue.
'SWAT' can be found hidden inside 'midshipmans watch'.
(Other definitions for swat that I've seen before include "Smack (a fly?)" , ".`Twas quite a blow" , "Hit hard, say fly, with something flat" , "land crushing blow" , "'Twas quite a smack" .)