Implied diplomacy protects one (5)
I believe the answer is:
tacit
'implied' is the definition.
('tacit' can be a synonym of 'implied')
'diplomacy protects one' is the wordplay.
'diplomacy' becomes 'tact' (I've seen this before).
'protects' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'tact' enclosing 'i' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Silently understood" , "Respect, admiration" , "Unspoken but understood" , "Understood without being directly expressed" , "Understood without being put into words" .)