Mum's at the bus terminus a lot (4)
I believe the answer is:
mass
'a lot' is the definition.
(I know that lot can be written as mass)
'mum's at the bus terminus' is the wordplay.
'mum's' becomes 'ma's' (ma can mean mother).
'at' means one lot of letters go next to another.
'the bus terminus' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'mas'+'s'='MASS'
Can you help me to learn more?
(Other definitions for mass that I've seen before include "Service; horde" , "13 [BULK]" , "Large number of people" , "Large amount, body" , "seasonal midnight service?" .)