Joined diplomat on date (8)
I believe the answer is:
attached
'joined' is the definition.
('attached' can be similar in meaning to 'joined')
'diplomat on date' is the wordplay.
'diplomat' becomes 'attache' (I've seen this before).
'on' says to put letters next to each other.
'date' becomes 'd' (common abbreviation eg in DOB, date of birth).
'attache'+'d'='ATTACHED'
(Other definitions for attached that I've seen before include "Married" , "Affixed" , "Stuck" , "Connected, associated" , "in a relationship" .)