Mum joins church club (4)
I believe the answer is:
mace
'club' is the definition.
(mace can mean a club or stick)
'mum joins church' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'joins' means one lot of letters go next to another.
'church' becomes 'ce' (Church of England).
'ma'+'ce'='MACE'
(Other definitions for mace that I've seen before include "Tear gas - spice" , "Disabling spray" , "something fragrant" , "Medieval weapon or kind of spice" , "Staff - ground nutmeg" .)