Terrific increase (5)
I believe the answer is:
swell
I believe this clue is a double definition.
'terrific' is the first definition.
(I know that terrific can be written as swell)
'increase' is the second definition.
(I know that swell has the is a more specific form of the action expand)
(Other definitions for swell that I've seen before include "Person of high social standing" , "marvellous" , "Distend" , "Rise into waves" , "get fat" .)