Diplomacy in charge - Scot is a strategist (9)
I believe the answer is:
tactician
'strategist' is the definition.
(I've seen this before)
'diplomacy in charge scot' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'scot' becomes 'ian'.
'tact'+'ic'+'ian'='TACTICIAN'
'is a' is the link.
(Other definitions for tactician that I've seen before include "One deploying forces in war" , "Strategist" , "Someone skilled in manoeuvres" , "General" , "Skilled planner" .)