A pain to accommodate dry official on mission (7)
I believe the answer is:
attache
'official on mission' is the definition.
(member of a diplomatic mission)
'a pain to accommodate dry' is the wordplay.
'pain' becomes 'ache' (ache is a kind of pain).
'to accommodate' means one lot of letters goes inside another.
'dry' becomes 'tt' (abbreviation for teetotal).
'a'+'ache'='aache'
'aache' going around 'tt' is 'ATTACHE'.
(Other definitions for attache that I've seen before include "Specialised envoy" , "Tea chat (anag.)" , "That ace (anag.)" , "Ambassador's aide" , "ambassador's assistant" .)