Nailed, say, with sensitivity (4)
I believe the answer is:
tact
'sensitivity' is the definition.
(being tactful is being sensitive)
'nailed say' is the wordplay.
'nailed' becomes 'tacked'.
'say' indicates a 'sounds like' (homophone) clue.
'tacked' sounds like 'TACT'.
'with' acts as a link.
(Other definitions for tact that I've seen before include "Understanding" , "Fine perception" , "Diplomacy or discretion" , "Consideration and sensitivity" , "measure of diplomacy?" .)