Best expert coming back in as reinforcement (6)
I believe the answer is:
toecap
'reinforcement' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'best expert coming back in' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'expert' becomes 'ace' (ace can informally mean very good).
'coming back' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'ace' backwards is 'eca'.
'top' going around 'eca' is 'TOECAP'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for toecap that I've seen before include "part of Oxford, perhaps" , "Protective cover on boot or shoe" , "Strengthener on front of boot, shoe" , "Protective front of boot" , "Boot reinforcement" .)