Everybody is in favour of a gasbag! (7)
I believe the answer is:
balloon
'a gasbag' is the definition.
(thesaurus)
'everybody is in favour' is the wordplay.
'everybody' becomes 'all' (all people).
'is in' indicates putting letters inside.
'favour' becomes 'boon' (I've seen this before).
'all' inserted into 'boon' is 'BALLOON'.
'of' acts as a link.
(Other definitions for balloon that I've seen before include "One blown up for party" , "Inflatable rubber bag, could be filled with hot air" , "Inflate" , "Bag filled with gas or air" , "Brandy glass" .)