Diplomacy is to adopt Charlie's strategy (7)
I believe the answer is:
tactics
'strategy' is the definition.
(I've seen this in another clue)
'diplomacy is to adopt charlie's' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'to adopt' indicates putting letters inside (inserted letters are taken in or adopted).
'charlie' becomes 'C' (phonetic alphabet: alpha, bravo, charlie etc.).
'is' going around 'c' is 'ics'.
'tact'+'ics'='TACTICS'
(Other definitions for tactics that I've seen before include "Art of organising battle" , "Actions planned to achieve an end" , "Manuvres" , "game plan" , "Ploys" .)