Cheerfully casual friends from south very quietly lying in cut grass (4-5)
I believe the answer is:
slap-happy
'cheerfully casual' is the definition.
(cheerfully casual or irresponsible)
'friends from south very quietly lying in cut grass' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'from south' shows that the letters should be reversed in order.
'very quietly' becomes 'pp' (musical abbreviation for pianissimo).
'lying in' means one lot of letters goes inside another.
'cut grass' becomes 'hay' (I've seen this before).
'pals' in reverse letter order is 'slap'.
'pp' inserted within 'hay' is 'happy'.
'slap'+'happy'='SLAP-HAPPY'
(Other definitions for slap-happy that I've seen before include "Boisterously" , "Cheerfully irresponsible" , "free with the greasepaint?" , "Silly - unmethodical - punch-drunk" .)