Ambassador has to deal with Top Gear (9)
I believe the answer is:
headdress
'top gear' is the definition.
(I've seen this before)
'ambassador has to deal with' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'has' means one lot of letters go next to another.
'to deal with' becomes 'address' ('address' can be a synonym of 'deal with'**).
'he'+'address'='HEADDRESS'
(Other definitions for headdress that I've seen before include "Top cover" , "Hat or tiara, say" , "top gear" , "capital cover" , "Ceremonial wear" .)