Swimmer with British papers (5)
I believe the answer is:
bream
'swimmer' is the definition.
(bream is a kind of aquatic vertebrate)
'with british papers' is the wordplay.
'with' means one lot of letters go next to another.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'papers' becomes 'ream' (a ream is a large amount of paper).
'b'+'ream' is 'BREAM'.
(Other definitions for bream that I've seen before include "Broad, thin fish" , "Fish - British classical guitarist, b. 1933" , "Amber kind of fish" , "River fish" , "Swimmer" .)