One mum remains (3)
I believe the answer is:
ash
'remains' is the definition.
(I've seen this before)
'one mum' is the wordplay.
'one' becomes 'a' (**).
'mum' becomes 'sh' (I've seen this in other clues).
'a'+'sh'='ASH'
(Other definitions for ash that I've seen before include "Tree; burned residue" , "Remains of conflagration" , "Hardwood" , "Fire's remains" , "-- Wednesday" .)