Increase - that's just great (5)
I believe the answer is:
swell
This clue is a double definition.
'increase' is the first definition.
(I know this)
'great' is the second definition.
(I know that great can be written as swell)
'that's just' is the link.
(Other definitions for swell that I've seen before include "Inflate or enlarge, like posh person" , "Grand" , "Person of high social standing" , "Grow larger; excellent" , "blowing top" .)