Cheerfully casual friends turned up slightly drunk (4-5)
I believe the answer is:
slap-happy
'cheerfully casual' is the definition.
(cheerfully casual or irresponsible)
'friends turned up slightly drunk' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'turned up' is a reversal indicator.
'slightly drunk' becomes 'happy' (happy can mean drunk).
'pals' backwards is 'slap'.
'slap'+'happy'='SLAP-HAPPY'
(Other definitions for slap-happy that I've seen before include "Boisterously" , "Silly - unmethodical - punch-drunk" , "free with the greasepaint?" , "Cheerfully irresponsible" .)