Clean with volcanic dust (4)
I believe the answer is:
wash
'clean' is the definition.
(washing is a kind of cleaning)
'with volcanic dust' is the wordplay.
'with' becomes 'w' (abbreviation).
'volcanic dust' becomes 'ash' (I've seen this before).
'w'+'ash'='WASH'
(Other definitions for wash that I've seen before include "Be credible" , "Brush with ink; launder" , "Perform ablutions" , "Bathe" , "Clean; dilute coat" .)