Become sunnier in resort, we hear (8)
I believe the answer is:
brighten
'become sunnier' is the definition.
'resort we hear' is the wordplay.
'resort' becomes 'brighton' (seaside resort).
'we hear' indicates a 'sounds like' (homophone) clue.
'brighton' sounds like 'BRIGHTEN'.
'in' acts as a link.
(Other definitions for brighten that I've seen before include "Provide illuminations" , "Polish" , "Berthing (anag.)" , "Liven up" , "Cheer up; clear up" .)