At home, a casual night with mum staying in can be dull (9)
I believe the answer is:
inanimate
'be dull' is the definition.
(I've seen this before)
'at home a casual night with mum staying in' is the wordplay.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'casual night' becomes 'nite' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'mum' becomes 'ma' (both can mean mother).
'staying in' means one lot of letters goes inside another.
'a'+'nite'='anite'
'anite' enclosing 'ma' is 'animate'.
'in'+'animate'='INANIMATE'
'can' is the link.
Can you help me to learn more?
(Other definitions for inanimate that I've seen before include "Showing no sign of life" , "Dull" , "dead" , "Not living" , "Unlikely to move" .)