Tree surgeon's first answer after rejecting party (5)
I believe the answer is:
balsa
'tree' is the definition.
(balsa is a kind of tree)
'surgeon's first answer after rejecting party' is the wordplay.
'first' indicates taking the first letters.
'answer' becomes 'a' (abbreviation eg in Q and A).
'after' is a charade indicator (letters next to each other).
'rejecting' is a reversal indicator.
'party' becomes 'lab' (abbreviation for Labour).
The first letter of 'surgeons' is 's'.
'lab' written backwards gives 'bal'.
's'+'a'='sa'
'sa' put after 'bal' is 'BALSA'.
(Other definitions for balsa that I've seen before include "lightweight boat-builder" , "Strong lightweight wood" , "Wood used by model makers" , "Something light" , "Very lightweight wood from tropical tree" .)