Swimmer wins National Final, making a comeback in contest (7)
I believe the answer is:
contend
'contest' is the definition.
(synonyms)
'swimmer wins national final making a comeback' is the wordplay.
'swimmer' becomes 'cod' (cod is a kind of aquatic vertebrate).
'wins' means one lot of letters goes inside another.
'national' becomes 'n' (abbreviation listed in Chambers).
'final' becomes 'net' (synonyms).
'making a comeback' is a reversal indicator.
'net' back-to-front is 'ten'.
'n'+'ten'='nten'
'cod' placed around 'nten' is 'CONTEND'.
'in' is the link.
(Other definitions for contend that I've seen before include "Engage in a struggle" , "Debate - contest - claim" , "Tend NCO (anag.)" , "Maintain" , "Compete, vie" .)