Figure a companion to be a swimmer (5)
I believe the answer is:
tench
'a swimmer' is the definition.
(tench is a kind of fish)
'figure a companion' is the wordplay.
'figure' becomes 'ten' (ten is a number or figure).
'a companion' becomes 'ch' (Companion of Honour).
'ten'+'ch'='TENCH'
'to be' is the link.
(Other definitions for tench that I've seen before include "something fishy" , "catch" , "Coarse fish" , "Fish of the carp family" , "Swimmer" .)