Fish with second fish (5)
I believe the answer is:
moray
'fish' is the definition.
(moray is a kind of fish)
'fish with second' is the wordplay.
'fish' becomes 'ray' (ray is a kind of fish).
'with' means one lot of letters go next to another.
'second' becomes 'mo' (both are short periods of time).
'ray' after 'mo' is 'MORAY'.
(Other definitions for moray that I've seen before include "is fishy" , "One fish" , "Firth" , "seen snaking near the coast?" , "Predator eel" .)