Breast firstly coated with leftover supreme (8)
I believe the answer is:
unbeaten
'supreme' is the definition.
'breast firstly coated with leftover' is the wordplay.
'firstly' says to take the initial letters.
'coated with' is an insertion indicator.
'leftover' becomes 'uneaten' (similar in meaning).
The initial letter of 'breast' is 'b'.
'b' inserted into 'uneaten' is 'UNBEATEN'.
(Other definitions for unbeaten that I've seen before include "Always top" , "not having suffered defeat" , "Never defeated" , "Always winning" , "Not punished" .)