Core muscle, with miles gone, stretched by character in Marathon (7)
I believe the answer is:
'core' is the definition.
'muscle with miles gone stretched by character in marathon' is the wordplay.
I cannot quite see how this works, but
'character' could be 'nu' (nu is a kind of letter of the alphabet) and 'nu' is found within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nucleus that I've seen before include "Central core of an atom" , "Heart of the Matter" , "hard core" , "Tiny central part of an atom" , "centrepiece?" .)