Yes, mum is in white (4)
I believe the answer is:
ashy
'white' is the definition.
(thesaurus)
'yes mum is in' is the wordplay.
'yes' becomes 'ay'.
'mum' becomes 'sh' (I've seen this in other clues).
'is in' means one lot of letters goes inside another.
'ay' enclosing 'sh' is 'ASHY'.
(Other definitions for ashy that I've seen before include "evidence of recent fire" , "Dusty" , "Pale grey" , "Grey-coloured" , "Like, eg, volcanic dust" .)