Swimmer turning over for medic (3)
I believe the answer is:
doc
'for medic' is the definition.
(doctor)
'swimmer turning' is the wordplay.
'swimmer' becomes 'cod' (type of fish).
'turning' says the letters should be written backwards.
'cod' reversed gives 'DOC'.
'over' is the link.
(Other definitions for doc that I've seen before include "Medic (familiar)" , "GP (informal)" , "Medical practitioner (abbr)" , "Holliday (in 17 24)" , "Fox perhaps" .)