Explain clue with detail endlessly broken down (9)
I believe the answer is:
elucidate
'explain' is the definition.
(elucidating is a kind of explaining)
'clue with detail endlessly broken down' is the wordplay.
'with' is a charade indicator (letters next to each other).
'endlessly' means to remove the last letter.
'broken down' indicates anagramming the letters.
'detail' with its last letter taken away is 'detai'.
'clue' after 'detai' is 'detaiclue'.
'detaiclue' anagrammed gives 'ELUCIDATE'.
(Other definitions for elucidate that I've seen before include "Make clear, explain" , "Shed light upon" , "Throw light on a problem, make clear (9)" , "Shed light on, clarify" , "Make clear and more comprehensible" .)