Loudly disapprove of new benefit (4)
I believe the answer is:
boon
'benefit' is the definition.
(I've seen this before)
'loudly disapprove of new' is the wordplay.
'loudly disapprove of' becomes 'boo' (noise which expresses disapproval).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'boo'+'n'='BOON'
(Other definitions for boon that I've seen before include "Beneficial thing" , "Favour, request" , "Benefit and advantage" , "Something beneficial - a blessing" , "Godsend" .)