Implied one is entering diplomacy (5)
I believe the answer is:
tacit
'implied' is the definition.
('tacit' can be a synonym of 'implied')
'one is entering diplomacy' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is entering' indicates putting letters inside.
'diplomacy' becomes 'tact' (I've seen this before).
'i' put inside 'tact' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Implied without being put into words" , "Implied without being said" , "Unrecorded" , "Implied, unstated" , "Implied but not directly expressed" .)