Ambassador interrupts fool with burnt residue (5)
I believe the answer is:
ashes
'burnt residue' is the definition.
(I've seen this before)
'ambassador interrupts fool' is the wordplay.
'ambassador' becomes 'HE' (His or Her Excellency).
'interrupts' is an insertion indicator.
'fool' becomes 'ass' (both can mean a stupid person).
'he' placed inside 'ass' is 'ASHES'.
'with' acts as a link.
(Other definitions for ashes that I've seen before include "Remnants of a fire" , "sports contest" , "Cricket honour in the fire" , "Remains from a cremation" , "Cricketing trophy - remains" .)