It's splendid to expand (5)
I believe the answer is:
swell
I believe this clue is a double definition.
'it's splendid' is the first definition.
(swell can informally mean excellent)
'to expand' is the second definition.
(swelling is a kind of expanding)
(Other definitions for swell that I've seen before include "Burgeon" , "'Increase in size, number etc (5)'" , "Man of high social standing" , "Heave of the sea's surface" , "First class" .)