Expression of regret for a shirt with grey edges (7)
I believe the answer is:
apology
'expression of regret' is the definition.
(synonyms)
'a shirt with grey edges' is the wordplay.
'shirt' becomes 'polo shirt' (polo shirt is a kind of shirt).
'with' means one lot of letters go next to another.
'edges' says to hollow out the word (remove centre letters) (I've seen 'edges of' mean this).
'grey' with its centre taken out is 'gy'.
'a'+'polo'+'gy'='APOLOGY'
'for' acts as a link.
(Other definitions for apology that I've seen before include "Regretful acknowledgement of a fault" , "defence" , "Excuse" , "Poor substitute" , "Expression of regret, very sorry" .)