Grass is new on old ship (4)
I believe the answer is:
nark
'grass' is the definition.
(I've seen this before)
'new on old ship' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'old ship' becomes 'ark' (as in Noah's ark).
'n'+'ark'='NARK'
'is' is the link.
(Other definitions for nark that I've seen before include "Annoy - informant" , "upset" , "Bother" , "Keep an eye" , "source of irritation" .)