A shirt unfortunately containing new stain (7)
I believe the answer is:
tarnish
'stain' is the definition.
(I know that stain can be written as tarnish)
'a shirt unfortunately containing new' is the wordplay.
'unfortunately' is an anagram indicator (a bad or unfortunate spelling).
'containing' indicates putting letters inside.
'new' becomes 'n' (abbreviation as in New Testament NT).
'a'+'shirt'='ashirt'
'ashirt' is an anagram of 'tarish'.
'tarish' placed around 'n' is 'TARNISH'.
(Other definitions for tarnish that I've seen before include "Discolouration of metal surface" , "Mountain lake" , "(Of metal) lose lustre" , "Cause to lose lustre, say silver" , "Stain or blemish, silver or reputation" .)