Grow larger (6)
'grow larger' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for expand that I've seen before include "Spread out" , "Dilate" , "Enlarge - stretch - become more friendly" , "Blow up" , "Swell" .)
(Other definitions for swell that I've seen before include "Inflate - fine" , "Become enlarged" , "Balloon - movement of the sea's surface" , "Heave of the sea's surface" , "blowing top" .)