GP brought back fish (3)
I believe the answer is:
doc
'gp' is the definition.
(I've seen this in other clues)
'back fish' is the wordplay.
'back' says the letters should be written in reverse.
'fish' becomes 'cod' (type of fish).
'cod' backwards is 'DOC'.
'brought' is the link.
(Other definitions for doc that I've seen before include "Fox perhaps" , "GP (colloq.)" , "Medic (familiar)" , "GP (informal)" , "bones" .)