"Tea" 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 understand how they can define each other.
'tea with mum's sister' is the wordplay.
'tea' becomes 't' (I've seen this in other clues).
'with' is a charade indicator (letters next to each other).
'mum's sister' becomes 'aunt' (I can't explain this - if you can you should believe this answer much more).
't'+'aunt'='TAUNT'
Can you help me to learn more?
(Other definitions for taunt that I've seen before include "Thing said to anger or wound" , "Mock provokingly" , "Sarcastic challenge" , "Mock, jeer" , "Jeer, deride" .)