Swimmer's main supporting structure crossing river (3,5)
I believe the answer is:
sea bream
'swimmer's' is the definition.
(sea bream is a kind of aquatic vertebrate)
'main supporting structure crossing river' is the wordplay.
'main' becomes 'sea' ('sea' can be a synonym of 'main').
'supporting structure' becomes 'beam' (beam is a kind of supporting structure**).
'crossing' is an insertion indicator.
'river' becomes 'r'.
'beam' enclosing 'r' is 'bream'.
'sea'+'bream'='SEA-BREAM'
(Other definitions for sea bream that I've seen before include "Fish - seem Arab (anag)" , "Edible marine fish" , "Fish, valued as food" .)