Young bird escaped to rear of waving heather (9)
I believe the answer is:
fledgling
'young bird' is the definition.
(fledgling is a kind of young bird)
'escaped to rear of waving heather' is the wordplay.
'escaped' becomes 'fled' (I've seen this before).
'to rear of' suggests the final letters.
'heather' becomes 'ling' ('ling' can be a synonym of 'heather').
The final letter of 'waving' is 'g'.
'fled'+'g'+'ling'='FLEDGLING'
(Other definitions for fledgling that I've seen before include "Trainee" , "Young one in flight" , "Young bird" .)