Mum, not married, sensible under different conditions (9)
I believe the answer is:
otherwise
'under different conditions' is the definition.
(I've seen this before)
'mum not married sensible' is the wordplay.
'mum' becomes 'mother' (I've seen this before).
'not' indicates named letters should be taken away.
'married' becomes 'm'.
'sensible' becomes 'wise'.
'mother' with 'm' removed is 'other'.
'other'+'wise'='OTHERWISE'
(Other definitions for otherwise that I've seen before include "Alternatively" , "White rose (anag.)" , "White rose seen in a different way" , "in different circumstances" , "Apart from that" .)