What’s in creel? Very small fish (5)
I believe the answer is:
elver
'small fish' is the definition.
(term for a young eel)
'what's in creel? very' is the wordplay.
'what's in' says the answer is hidden in the clue.
'ELVER' is hidden within 'creel very'.
(Other definitions for elver that I've seen before include "A young 11" , "Slippery youngster" , "Immature fish" , "Young, migrating eel" , "Young migratory fish" .)