The excuse when ambassador is let off crime (7)
I believe the answer is:
treason
'crime' is the definition.
(treason is a kind of crime)
'the excuse' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'excuse' becomes 'reason' (I've seen this before).
't'+'reason'='TREASON'
'when ambassador is let off' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for treason that I've seen before include "LEse-majestE" , "Senator (anag)" , "Serious crime" , "Crime agains one's country" , "One rats (anag)" .)