Order ambassador to infiltrate elite (6)
I believe the answer is:
behest
'order' is the definition.
(I've seen this before)
'ambassador to infiltrate elite' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'to infiltrate' means one lot of letters goes inside another.
'elite' becomes 'best'.
'he' placed within 'best' is 'BEHEST'.
(Other definitions for behest that I've seen before include "Command - Thebes (anag)" , "(At someone's) command" , "Order" , "Command, bidding" .)