Understood one to be in diplomacy (5)
I believe the answer is:
tacit
'understood' is the definition.
(thesaurus)
'one to be in diplomacy' is the wordplay.
'one' becomes 'i' (Roman numeral).
'to be in' means one lot of letters goes inside another.
'diplomacy' becomes 'tact' (I've seen this before).
'i' put into 'tact' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Implied, unstated" , "Implied or understood, not put into words" , "Unvoiced" , "Implicitly understood" , "Silent but implied" .)