Swimmer offers resistance in support of gymnast (5)
I believe the answer is:
bream
'swimmer' is the definition.
(bream is a kind of aquatic vertebrate)
'resistance in support of gymnast' is the wordplay.
'resistance' becomes 'r' (abbreviation).
'in' means one lot of letters goes inside another.
'support of gymnast' becomes 'beam'.
'r' put within 'beam' is 'BREAM'.
'offers' acts as a link.
(Other definitions for bream that I've seen before include "Amber (anag) - fish" , "Fish - English guitarist" , "Broad, thin fish" , "River fish" , "Fish of the carp family" .)