Join diplomat, not English (6)
I believe the answer is:
attach
'join' is the definition.
(attaching is a kind of joining)
'diplomat not english' is the wordplay.
'diplomat' becomes 'attache' (I've seen this before).
'not' suggests deleting specific letters.
'english' becomes 'e' (abbreviation).
'attache' with 'e' removed is 'ATTACH'.
(Other definitions for attach that I've seen before include "Bind, fasten" , "Join; seize (legal)" , "'Affix, fasten (6)'" , "Tie, bind to" , "Stick" .)