Little fish always grabs line (5)
I believe the answer is:
elver
'little fish' is the definition.
(young eel)
'always grabs line' is the wordplay.
'always' becomes 'ever' (synonyms).
'grabs' indicates putting letters inside.
'line' becomes 'l' (used when specifying particular lines from a poem).
'ever' placed around 'l' is 'ELVER'.
(Other definitions for elver that I've seen before include "Little fish" , "Immature eel" , "Young slippery fish" , "Young migratory fish" , "A young 11" .)