Wonderful to expand (5)
I believe the answer is:
swell
I believe this is a double definition.
'wonderful' is the first definition.
(swell can mean excellent)
'expand' is the second definition.
(I know that swell is a more specific form of the action expand)
'to' acts as a link.
(Other definitions for swell that I've seen before include "Man of high social standing" , "Grand" , "Distend" , "Enlargement when liquid accumulates" , "Dilate - jolly good!" .)