Scheme needing diplomacy by one Conservative (6)
I believe the answer is:
tactic
'scheme' is the definition.
(thesaurus)
'diplomacy by one conservative' is the wordplay.
'diplomacy' becomes 'tact' (I've seen this before).
'by' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'tact'+'i'+'c'='TACTIC'
'needing' acts as a link.
(Other definitions for tactic that I've seen before include "Stratagem, procedure" , "A manoeuvre or planned action" , "Military means to an end" , "Planned action/strategy" , "Device" .)