Time with mum's sister Barb (5)
I believe the answer is:
taunt
'barb' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'time with mum's sister' is the wordplay.
'time' becomes 't' (abbreviation).
'with' is a charade indicator (letters next to each other).
'mum's sister' becomes 'aunt' (I can't justify this - if you can you should give a lot more credence to this answer).
't'+'aunt'='TAUNT'
Can you help me to learn more?
(Other definitions for taunt that I've seen before include "Mock provokingly" , "element of teasing" , "Insult" , "Make fun of" , "Rag, goad" .)