Mum, not married, sensible apart from that (9)
I believe the answer is:
otherwise
'apart from that' is the definition.
(I've seen this before)
'mum not married sensible' is the wordplay.
'mum' becomes 'mother' (I've seen this before).
'not' suggests deleting specific letters.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'sensible' becomes 'wise' (I've seen this in another clue).
'mother' with 'm' taken out is 'other'.
'other'+'wise'='OTHERWISE'
(Other definitions for otherwise that I've seen before include "Alternatively" , "White rose (anag.)" , "Differently - or else!" , "Under different conditions" , "In other respects or ways" .)