One can do anything as Whitehall diplomats essentially decline deals (4-2-3-6)
I believe the answer is:
jack-of-all-trades
'one can do' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'anything as whitehall diplomats essentially decline deals' is the wordplay.
I cannot quite see how this works, but
'decline' could be 'fall' (synonyms) and 'fall' is present in the answer.
'deals' could be 'trades' ('trade' can be a synonym of 'deal') and 'trades' is present in the answer.
The remaining letters 'jacko' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for jack-of-all-trades that I've seen is " will he do any job?".)