Swimmer with a bad back (3)
I believe the answer is:
dab
'swimmer' is the definition.
(dab is a type of fish)
'a bad back' is the wordplay.
'back' says the letters should be written backwards.
'bad' back-to-front is 'DAB'.
'with' acts as a link.
(Other definitions for dab that I've seen before include "Fingerprint - fish" , "Bad sort of small fish" , "Apply light touches of colour to the fish" , "Apply small amounts (usually a liquid) to a surface" , "Flounder" .)