Unspoken understanding that one's engaged in diplomacy (5)
I believe the answer is:
tacit
'unspoken understanding that' is the definition.
'tacit' can be an answer for 'unspoken' (I've seen this in another clue). I'm unsure of the remainder of the definition.
'one's engaged in diplomacy' is the wordplay.
'one' becomes 'i' (Roman numeral).
'engaged in' means one lot of letters goes inside another.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'i' put inside 'tact' is 'TACIT'.
Can you help me to learn more?
(Other definitions for tacit that I've seen before include "Understood, implicit" , "without speaking" , "Silent - understood" , "Implied -- inferred" , "Understood, not spoken or played" .)