Squiffy setter's besetting sin (5)
I believe the answer is:
merry
'squiffy' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I don't see?
'setter's besetting sin' is the wordplay.
'setter's' becomes 'my' (belonging to the crossword setter).
'besetting' indicates putting letters inside.
'sin' becomes 'err' (to sin is to err morally).
'my' enclosing 'err' is 'MERRY'.
Can you help me to learn more?
(Other definitions for merry that I've seen before include "Seasons 24" , "Drunk - happy" , "Full of fun and gaiety" , "Operetta by 11" , "Fjull of fun and gaiety" .)