Young bird stuck at first in trap on heather (8)
I believe the answer is:
nestling
'young bird' is the definition.
(nestling is a kind of young bird)
'stuck at first in trap on heather' is the wordplay.
'at first' suggests taking the first letters.
'in' is an insertion indicator.
'trap' becomes 'net' (net is a kind of trap).
'on' means one lot of letters go next to another.
'heather' becomes 'ling' (synonyms).
The initial letter of 'stuck' is 's'.
's' placed within 'net' is 'nest'.
'nest'+'ling'='NESTLING'
(Other definitions for nestling that I've seen before include "not yet put out" , "getting under duvet?" , "Baby bird" , "Little bird" , "Unfledged bird" .)