Diplomacy initially is clearly expedient (6)
I believe the answer is:
tactic
'expedient' is the definition.
(I've seen this in another clue)
'diplomacy initially is clearly' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'initially' says to take the initial letters.
The initial letters of 'is clearly' is 'ic'.
'tact'+'ic'='TACTIC'
(Other definitions for tactic that I've seen before include "Action aimed at a specific end" , "Skilful move or plan" , "A TT C-i-C (anag.)" , "Action/strategy to achieve a specific end" , "Device" .)