Nervous giggle (6)
I believe the answer is:
titter
'nervous giggle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for titter that I've seen before include "Suppressed snigger" , "A high-pitched giggle" , "Stifled giggle" , "Half-suppressed laugh" , "Stifled laugh" .)