Mostly offensive, he may be a member of the diplomatic corps (7)
I believe the answer is:
attache
'a member of the diplomatic corps' is the definition.
(attache can mean an embassy worker)
'mostly offensive he' is the wordplay.
'mostly' means to remove the last letter.
'offensive' becomes 'attack' (an attack is a military offensive).
'attack' with its last letter taken away is 'attac'.
'attac'+'he'='ATTACHE'
'may be' is the link.
(Other definitions for attache that I've seen before include "One on embassy staff" , "Man with a mission" , "Junior official" , "Specialist on embassy staff" , "Embassy worker" .)