Get together when mum's in (5)
I believe the answer is:
amass
'get together' is the definition.
(I've seen this before)
'when mum's in' is the wordplay.
'when' becomes 'as'.
'mum's' becomes 'mas' ('ma' can be a synonym of 'mum').
'in' indicates putting letters inside.
'as' enclosing 'mas' is 'AMASS'.
(Other definitions for amass that I've seen before include "store" , "Assemble" , "Sodium chloride" , "Stock up on" , "Accumulate, heap up" .)