Silent diplomacy involving one (5)
I believe the answer is:
tacit
'silent' is the definition.
(I know that silent can be written as tacit)
'diplomacy involving one' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'involving' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'tact' going around 'i' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Understood, implicit" , "Silent - understood" , "Implicitly understood" , "Not said though understood" , "Implied or understood, not put into words" .)