Mum, it seems, married someone else! (6)
I believe the answer is:
mother
'mum' is the definition.
(I've seen this before)
'someone else' is the wordplay.
'someone' becomes 'm' (I am not sure about this - if you are sure you should believe this answer much more).
'else' becomes 'other' (synonyms).
'm'+'other'='MOTHER'
'it seems married' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for mother that I've seen before include "One who pours the tea?" , "She gave birth to her Tom" , "Female parent" , "Dam" , "She gives birth" .)