Grass is pretty when cut to half length (3)
I believe the answer is:
rat
'grass' is the definition.
(both can mean a snitch or informer)
'pretty when cut to half length' is the wordplay.
'pretty' becomes 'rather' ('rather' can be a synonym of 'pretty').
'when cut to half length' suggests halving the letters.
'rather' cut in half is 'RAT'.
'is' is the link.
(Other definitions for rat that I've seen before include "Betrayer" , "Contemptible person" , "Rodent - deserter" , "Despicable person" , "Cad" .)