Ambassador covered in strange nasal discharge (5)
I believe the answer is:
rheum
'nasal discharge' is the definition.
(I've seen this before)
'ambassador covered in strange' is the wordplay.
'ambassador' becomes 'HE' (His or Her Excellency).
'covered in' means one lot of letters goes inside another.
'strange' becomes 'rum' (similar in meaning).
'he' inserted inside 'rum' is 'RHEUM'.
(Other definitions for rheum that I've seen before include "that's rather snotty?" , "runny nose" , "Watery stuff" , "sign of illness" , "cold once upon a time" .)