Shirt — an awful stain (7)
I believe the answer is:
tarnish
'stain' is the definition.
(tarnishing is a kind of staining)
'shirt an awful' is the wordplay.
'awful' indicates anagramming the letters.
'shirt'+'an'='shirtan'
'shirtan' with letters rearranged gives 'TARNISH'.
(Other definitions for tarnish that I've seen before include "Stain like silver" , "Stain or blemish, silver or reputation" , "Cause to lose lustre, silver or reputation" , "Discolouration of metal surface" , "Become dull or blemished" .)