Sign of amusement (4)
I believe the answer is:
grin
'sign of amusement' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for grin that I've seen before include "Beam" , "Look pleased" , "A broad smile showing the teeth" , "11 smile" , "Facial expression showing amusement or pleasure" .)