How to explain a diet clue, perhaps (9)
I believe the answer is:
elucidate
'how to explain' is the definition.
(I know that elucidate is a more specific form of the action clarify)
'a diet clue perhaps' is the wordplay.
'perhaps' indicates anagramming the letters.
'a'+'diet'+'clue'='adietclue'
'adietclue' with letters rearranged gives 'ELUCIDATE'.
(Other definitions for elucidate that I've seen before include "Make clear and more comprehensible" , "Make clear, explain" , "Shed light on, clarify" , "Shed light upon" , "Throw light on a problem, make clear (9)" .)