Cryptic clue: Sun's core (7)
I believe the answer is:
nucleus
'core' is the definition.
(I know that core can be written as nucleus)
'cryptic clue sun's' is the wordplay.
'cryptic' indicates anagramming the letters (a cryptic or obscure order of the letters).
'clue'+'sun'='cluesun'
'cluesun' anagrammed gives 'NUCLEUS'.
(Other definitions for nucleus that I've seen before include "centrepiece?" , "central component" , "Central part of a cell" , "Tiny central part of an atom" , "Heart of the Matter" .)