Unlike mum not having married (5)
I believe the answer is:
other
'unlike' is the definition.
(thesaurus)
'mum not having married' is the wordplay.
'mum' becomes 'mother' (I've seen this before).
'not having' indicates named letters should be taken away.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'mother' with 'm' removed is 'OTHER'.
(Other definitions for other that I've seen before include "Very unusual" , "Alternative - unrelated" , "Remaining" , "not like you" , "One may be significant" .)