Silly person, the ambassador in dress (6)
I believe the answer is:
clothe
'dress' is the definition.
(I know that dress can be written as clothe)
'silly person the ambassador' is the wordplay.
'silly person' becomes 'clot'.
'the ambassador' becomes 'HE' (His or Her Excellency).
'clot'+'he'='CLOTHE'
'in' is the link.
(Other definitions for clothe that I've seen before include "robe" , "Dress (someone)" , "Provide with garments" , "Put garments onto" , "Attire" .)