Cleanse with (initially) grey powder? (4)
I believe the answer is:
wash
'cleanse' is the definition.
(washing is a kind of cleansing)
'with initially grey powder?' is the wordplay.
'initially' suggests taking the first letters.
'grey powder?' becomes 'ash' (ashes are grey).
The initial letter of 'with' is 'w'.
'w'+'ash'='WASH'
(Other definitions for wash that I've seen before include "Of animals, cleanse by licking" , "Bear investigation" , "Clean with water" , "Scrub" , "The ----, inlet of the North Sea" .)