Swimmer, always about 50 (5)
I believe the answer is:
elver
'swimmer' is the definition.
(elver is a kind of fish)
'always about 50' is the wordplay.
'always' becomes 'ever' (synonyms).
'about' is an insertion indicator.
'50' becomes 'l' (roman numeral).
'ever' placed around 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "One may see one upriver" , "Immature fish" , "Young migratory fish" , "Young swimmer" , "slippery little thing" .)