Punch-drunk, but right cheerful (9)
I believe the answer is:
slaphappy
'punch-drunk' is the definition.
(synonyms)
'right cheerful' is the wordplay.
'right' becomes 'slap' (I've seen this before).
'cheerful' becomes 'happy' (similar in meaning).
'slap'+'happy'='SLAPHAPPY'
'but' acts as a link.
(Other definitions for slaphappy that I've seen before include "careless" , "Cheerfully casual" , "Cheerfully irresponsible" , "Silly -- unmethodical -- punch-drunk" .)