Conservative leader's rubbish without diplomatic skill (4)
I believe the answer is:
tact
'diplomatic skill' is the definition.
(I've seen this before)
'conservative leader's rubbish without' is the wordplay.
'leader' suggests taking the first letters.
'rubbish' becomes 'tat' (low-quality items or material).
'without' indicates putting letters inside.
The initial letter of 'conservative' is 'c'.
'c' inserted within 'tat' is 'TACT'.
(Other definitions for tact that I've seen before include "Diplomacy or discretion" , "measure of diplomacy?" , "Diplomatic sensitivity" , "Sensitivity in dealing with people" , "Consideration in dealing with others" .)