Knock vale swathed in grass (3-1-3)
I believe the answer is:
rat-a-tat
'knock' is the definition.
(I've seen this before)
'vale swathed in grass' is the wordplay.
'vale' becomes 'ta-ta' (ta-ta can mean bye - 'vale' is farewell in Latin).
'swathed in' is an insertion indicator (swathe can mean to wrap).
'grass' becomes 'rat' (both can mean a snitch or informer).
'tata' put within 'rat' is 'RAT-A-TAT'.
(Other definitions for rat-a-tat that I've seen before include "Rapping sound" , "Knocking sound" , "Noise of knocking" .)