Duck in grass moving quietly? (8)
I believe the answer is:
stealing
'moving quietly?' is the definition.
'duck in grass' is the wordplay.
'duck' becomes 'teal' (teal is a type of duck).
'in' is an insertion indicator.
'grass' becomes 'sing' (I've seen this before).
'teal' put into 'sing' is 'STEALING'.
(Other definitions for stealing that I've seen before include "Taking illegally" , "Robbing" , "Thieving" , "Theft" , "Illegally removing" .)