Ambassador involved in crash may have a grievance (3,4)
I believe the answer is:
the hump
'have a grievance' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'ambassador involved in crash' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'involved' indicates putting letters inside.
'in crash' becomes 'thump' (I can't justify this - if you can you should give a lot more credence to this answer).
'he' going within 'thump' is 'THE HUMP'.
'may' acts as a link.
Can you help me to learn more?
(Another definition for the hump that I've seen is " Fit of annoyance".)