Garment covered in grass must be put right (9)
I believe the answer is:
redressed
'put right' is the definition.
(I've seen this before)
'garment covered in grass' is the wordplay.
'garment' becomes 'dress' (dress is a kind of garment).
'covered in' means one lot of letters goes inside another.
'grass' becomes 'reed' (type of grass plant).
'dress' inserted within 'reed' is 'REDRESSED'.
'must be' is the link.
(Other definitions for redressed that I've seen before include "put right" , "Remedied" , "Made amends" , "Set right, often the balance" , "having gone straight" .)