Not expressed by one in diplomacy (5)
I believe the answer is:
tacit
'not expressed' is the definition.
(I've seen this in another clue)
'one in diplomacy' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'i' going inside 'tact' is 'TACIT'.
'by' is the link.
(Other definitions for tacit that I've seen before include "Understood without being put into words" , "Understood, not spoken or played" , "Implied, unstated" , "Implicitly understood" , "Implied -- inferred" .)