Mother had a mission (6)
I believe the answer is:
teresa
'mission' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I don't see?
'mother had a' is the wordplay.
'mother' becomes 'teres' (I can't explain this - if you can you should give a lot more credence to this answer).
'had' says to put letters next to each other.
'teres'+'a'='TERESA'
Can you help me to learn more?
(Other definitions for teresa that I've seen before include "Nun who was dedicated to helping the poor of Calcutta" , "Mother of Calcutta, 1910-97" , "Late nun, dedicated to helping the poor in India" , "Mother...saintly nun of Calcutta" , "Girl's name" .)