Second girl for fish (5)
I believe the answer is:
skate
'fish' is the definition.
(skate is a kind of fish)
'second girl' is the wordplay.
'second' becomes 's' (common abbreviation).
'girl' becomes 'kate'.
's'+'kate'='SKATE'
'for' acts as a link.
(Other definitions for skate that I've seen before include "move around ice" , "Cartilaginous fish" , "Edible marine fish" , "A ray; slide smoothly" , "Large edible ray" .)