Married expert joined staff (4)
I believe the answer is:
mace
'staff' is the definition.
(mace is a kind of staff)
'married expert joined' is the wordplay.
'married' becomes 'm'.
'expert' becomes 'ace' (ace is a kind of expert).
'joined' says to put letters next to each other.
'm'+'ace'='MACE'
(Other definitions for mace that I've seen before include "Came to get ceremonial staff" , "Tear gas - spice" , "Disabling spray" , "Staff - ground nutmeg" , "Ceremonial staff or kind of spice" .)