Pitch up on the grass (3)
I believe the answer is:
rat
'the grass' is the definition.
(both can mean a snitch or informer)
'pitch up' is the wordplay.
'pitch' becomes 'tar' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'tar' in reverse letter order is 'RAT'.
'on' is the link.
(Other definitions for rat that I've seen before include "Unpopular rodent" , "Treacherous person" , "one who betrayed" , "Unpleasant character" , "Despicable person" .)