I engaged in diplomacy without speaking (5)
I believe the answer is:
tacit
'without speaking' is the definition.
(I've seen this before)
'i engaged in diplomacy' is the wordplay.
'engaged in' means one lot of letters goes inside another.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'i' inserted inside 'tact' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Attic? (anag)" , "Implied -- inferred" , "Implied but not directly expressed" , "Silently understood" , "Silent - understood" .)