Cheer up when you mention the seaside (8)
I believe the answer is:
brighten
'cheer up' is the definition.
(I've seen this before)
'you mention the seaside' is the wordplay.
I cannot really see how this works, but
an anagram of 'the' is 'hte' which is within the answer.
This may be the basis of clue (or it may be nonsense).
'when' is the link.
Can you help me to learn more?
(Other definitions for brighten that I've seen before include "Berthing (anag.)" , "Cheer up; clear up" , "Illuminate, cheer up" , "Provide illuminations" , "Liven up" .)