Downgrading on diplomatic mission (10)
I believe the answer is:
relegation
'downgrading on diplomatic mission' is the definition.
I can't tell whether this defines the answer.
'on diplomatic mission' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'diplomatic mission' becomes 'legation' (legation is a kind of diplomatic mission).
're'+'legation'='RELEGATION'
Can you help me to learn more?
(Other definitions for relegation that I've seen before include "that's going down" , "Lowering of status" , "Transference to a lower league" , "Demotion, to a lower league" .)