Joined on a weekday (3)
I believe the answer is:
wed
This clue is a double definition.
'joined' is the first definition.
(thesaurus)
'weekday' is the second definition.
(I know that Wed is a type of weekday)
'on a' is the link.
(Other definitions for wed that I've seen before include "Take in marriage" , "Unite closely" , "united" , "Joined in the dew" , "Married" .)