Increase rapidly when everyone's in favour (7)
I believe the answer is:
balloon
'increase rapidly' is the definition.
(I've seen this before)
'everyone's in favour' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'in' means one lot of letters goes inside another.
'favour' becomes 'boon' (I've seen this before).
'all' going within 'boon' is 'BALLOON'.
'when' is the link.
(Other definitions for balloon that I've seen before include "In which Charlie Brown expressed himself" , "Become inflated" , "Inflatable rubber bag, could be filled with hot air" , "Inflatable - rapidly expand" , "See 21" .)