Not said to be one taken in by diplomacy (5)
I believe the answer is:
tacit
'not said' is the definition.
(I've seen this in another clue)
'one taken in by diplomacy' is the wordplay.
'one' becomes 'I' (Roman numeral).
'taken in by' indicates putting letters inside.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'i' going within 'tact' is 'TACIT'.
'to be' acts as a link.
(Other definitions for tacit that I've seen before include "Unrecorded" , "Aircraft engine casing" , "Silent - understood" , "Understood without being put into words" , "Implied without being put into words" .)