It’s great to expand (5)
I believe the answer is:
swell
I believe this clue is a double definition.
'it's great' is the first definition.
('swell' can be a synonym of 'great')
'expand' is the second definition.
(swelling is a kind of expanding)
'to' is the link.
(Other definitions for swell that I've seen before include "Movement of the surface of the sea" , "Bulge; a nob" , "Bulge; a toff" , "Fop" , "slow rise and fall" .)