Fish always left inside (5)
I believe the answer is:
elver
'fish' is the definition.
(elver is a kind of fish)
'always left inside' is the wordplay.
'always' becomes 'ever' (synonyms).
'left' becomes 'l' (common abbreviation).
'inside' indicates putting letters inside.
'ever' placed around 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "One may see one upriver" , "Immature eel" , "Young swimmer" , "Slippery youngster" , "Young migratory fish" .)