Wave to balloon (5)
I believe the answer is:
swell
This clue is a double definition.
'wave' is the first definition.
(I know that swell is a type of wave)
'balloon' is the second definition.
(I've seen this in another clue)
'to' is the link.
(Other definitions for swell that I've seen before include "slow rise and fall" , "mushroom" , "Expand abnormally" , "Enlarge; sea movement" , "Person of high social standing" .)