Early bird succeeded coming in to trap fish (8)
I believe the answer is:
nestling
'early bird' is the definition.
(I've seen this in another clue)
'succeeded coming in to trap fish' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'coming in to' means one lot of letters goes inside another.
'trap' becomes 'net' (net is a type of trap).
'fish' becomes 'ling' (ling is a kind of fish).
's' put into 'net' is 'nest'.
'nest'+'ling'='NESTLING'
(Other definitions for nestling that I've seen before include "Young bird - snuggling" , "not yet put out" , "getting under duvet?" , "Unfledged bird" , "Settling in comfortably, against someone perhaps" .)