Fit in with Heather's budding fliers (9)
I believe the answer is:
nestlings
'budding fliers' is the definition.
'nestlings' can be an answer for 'fliers' (I have seen 'Young birds' mean 'nestlings' so perhaps 'birds' could also mean 'nestlings'). I'm unsure of the 'budding' bit.
'fit in with heather's' is the wordplay.
'fit in' becomes 'nest' (I've seen this in another clue).
'with' says to put letters next to each other.
'heather's' becomes 'ling's' ('ling' can be a synonym of 'heather').
'nest'+'lings'='NESTLINGS'
Can you help me to learn more?
(Other definitions for nestlings that I've seen before include "Mummy bird has them" , "Baby birds" , "Young birds" , "brood" .)