Cheerful ape - but weird (6)
I believe the answer is:
upbeat
'cheerful' is the definition.
('upbeat' can be a synonym of 'cheerful')
'ape but weird' is the wordplay.
'weird' indicates anagramming the letters.
'ape'+'but'='apebut'
'apebut' is an anagram of 'UPBEAT'.
(Other definitions for upbeat that I've seen before include "Cheerfully optimistic" , "Cheerful and encouraging, like lively music" , "maestro communicates this to orchestra" , "bright and cheery" , "signal from sergeant, say, at end of bar?" .)