Diplomacy by one about second strategy (7)
I believe the answer is:
tactics
'strategy' is the definition.
(I've seen this in another clue)
'diplomacy by one about second' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'by' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'about' becomes 'c' (abbreviation for circa).
'second' becomes 's' (common abbreviation).
'tact'+'i'+'c'+'s'='TACTICS'
(Other definitions for tactics that I've seen before include "Art of organising battle" , "Use of available means to reach an end" , "Art of military manoeuvre" , "Skill of using available means to reach an end" , "Art of disposing military forces" .)