Yours truly involved in diplomacy, it’s understood (5)
I believe the answer is:
tacit
'understood' is the definition.
(I know that understood can be written as tacit)
'yours truly involved in diplomacy' is the wordplay.
'yours truly' becomes 'I' (the person that set this crossword).
'involved in' indicates putting letters inside.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'i' going within 'tact' is 'TACIT'.
'it's' is the link.
(Other definitions for tacit that I've seen before include "Understood without being put into words" , "Implied, not spoken" , "Implied, unstated" , "Implied but not directly expressed" , "Unvoiced" .)