Sweet expression of amusement with a part of the mouth at work (8)
I believe the answer is:
lollipop
'sweet' is the definition.
(lollipop is a kind of sweet)
'expression of amusement with a part of the mouth at work' is the wordplay.
'expression of amusement' becomes 'lol' (abbreviation for 'laugh out loud').
'with' is a charade indicator (letters next to each other).
'a part of the mouth' becomes 'lip' ('lip' is part of 'mouth').
'at' means one lot of letters go next to another.
'work' becomes 'op' (common abbreviation for 'opus').
'lol'+'lip'+'op'='LOLLIPOP'
(Other definitions for lollipop that I've seen before include "Sweetmeat" , "Confection on a stick" , "Popsicle" , "Sweet on a stick" , "sucker" .)