Birds start to whistle in trees (8)
I believe the answer is:
swallows
'birds' is the definition.
(swallow is a kind of bird)
'start to whistle in trees' is the wordplay.
'start to' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'trees' becomes 'sallows' (sallow is a kind of tree).
The first letter of 'whistle' is 'w'.
'w' placed within 'sallows' is 'SWALLOWS'.
(Other definitions for swallows that I've seen before include "Seasonal flyers" , "Small swift birds" , "Small migratory birds" , "migrants regularly taking flight" , "Strange slow laws apply to birds" .)