A shirt spoilt - hide new stain (7)
I believe the answer is:
tarnish
'stain' is the definition.
(tarnishing is a kind of staining)
'a shirt spoilt hide new' is the wordplay.
'spoilt' indicates anagramming the letters.
'hide' is an insertion indicator.
'new' becomes 'n'.
'a'+'shirt'='ashirt'
'ashirt' is an anagram of 'tarish'.
'tarish' enclosing 'n' is 'TARNISH'.
(Other definitions for tarnish that I've seen before include "Go dull" , "Take the shine off" , "Stain or blemish, silver or reputation" , "Film" , "(Of metal) lose lustre" .)