Silent diplomacy involving Irish leader (5)
I believe the answer is:
tacit
'silent' is the definition.
(synonyms)
'diplomacy involving irish leader' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'involving' indicates putting letters inside.
'leader' says to take the initial letters.
The first letter of 'irish' is 'i'.
'tact' placed around 'i' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Unrecorded" , "Silent - understood" , "Not said though understood" , "Implied, not spoken" , "Implied without being put into words" .)