... mum's sister acquiring second volume (6)
I believe the answer is:
amount
'volume' is the definition.
(I've seen this in another clue)
'mum's sister acquiring second' is the wordplay.
'mum's sister' becomes 'aunt' (I can't justify this - if you can you should give a lot more credence to this answer).
'acquiring' indicates putting letters inside.
'second' becomes 'mo' (both are short periods of time).
'aunt' placed around 'mo' is 'AMOUNT'.
Can you help me to learn more?
(Other definitions for amount that I've seen before include "Add up or be equivalent to" , "Measure, quantity" , "Supply" , "Quantity, sum" , "Sum of money" .)