Cheer up, just enthralled by Scottish mountain (8)
I believe the answer is:
brighten
'cheer up' is the definition.
(I've seen this before)
'just enthralled by scottish mountain' is the wordplay.
'just' becomes 'right' (associated in meaning).
'enthralled by' indicates putting letters inside ('enthral' can mean 'capture').
'scottish mountain' becomes 'ben' (Scottish term for a peak, eg Ben Nevis).
'right' inserted inside 'ben' is 'BRIGHTEN'.
(Other definitions for brighten that I've seen before include "Cheer up; clear up" , "Polish" , "Liven up" , "Berthing (anag.)" , "Provide illuminations" .)