Swimmer dealt a blow in flank (7)
I believe the answer is:
whiting
'swimmer' is the definition.
(whiting is a kind of fish)
'dealt a blow in flank' is the wordplay.
'dealt a blow' becomes 'hit'.
'in' is an insertion indicator.
'flank' becomes 'wing' (I've seen this before).
'hit' put within 'wing' is 'WHITING'.
(Other definitions for whiting that I've seen before include "Small sea fish or ground chalk for painting" , "Edible marine fish" , "North Atlantic food fish" , "[SOMETHING FISHY]" , "Small fish related to cod" .)