Nailed, say, with diplomacy (4)
I believe the answer is:
tact
'diplomacy' is the definition.
(tact can mean diplomacy or sensitivity)
'nailed say' is the wordplay.
'nailed' becomes 'tacked'.
'say' shows a homophone (sound like).
'tacked' sounds like 'TACT'.
'with' acts as a link.
(Other definitions for tact that I've seen before include "Sensitivity in dealings" , "Sensitivity, diplomacy" , "Fine perception" , "Skill in dealing with people" , "Diplomacy or discretion" .)