Regular guy drinking leaves merry (6)
I believe the answer is:
jocose
'merry' is the definition.
(I've seen this in another clue)
'regular guy drinking leaves' is the wordplay.
'regular guy' becomes 'joe' (I've seen this in another clue).
'drinking' is an insertion indicator (inserted letters are consumed or drunk).
'leaves' becomes 'cos' (cos lettuce leaves).
'joe' going around 'cos' is 'JOCOSE'.
(Other definitions for jocose that I've seen before include "Jesting and playful" , "Playful, making jokes" , "Playful and joking" , "Full of jokes" , "Facetious" .)