Cooked bun tucked away is never surpassed (8)
I believe the answer is:
unbeaten
'never surpassed' is the definition.
(I've seen this before)
'cooked bun tucked away' is the wordplay.
'cooked' indicates an anagram (letters cooked into a new form).
'tucked away' becomes 'eaten' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bun' is an anagram of 'unb'.
'unb'+'eaten'='UNBEATEN'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for unbeaten that I've seen before include "Victorious" , "there's nothing better" , "Not overcome" , "never surpassed" , "Never defeated" .)