Papers supporting British swimmer (5)
I believe the answer is:
bream
'swimmer' is the definition.
(bream is a kind of aquatic vertebrate)
'papers supporting british' is the wordplay.
'papers' becomes 'ream' (a ream is a large amount of paper).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'british' becomes 'b' (abbreviation e.g. in BBC).
'ream' after 'b' is 'BREAM'.
(Other definitions for bream that I've seen before include "Broad, thin fish" , "Food fish" , "Fish - British classical guitarist, b. 1933" , "Amber (anag) - fish" , "Amber kind of fish" .)