Heartless Conservative veteran (4)
I believe the answer is:
cold
'heartless' is the definition.
(I've seen this in another clue)
'conservative veteran' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'veteran' becomes 'old' (veteran can mean old or former).
'c'+'old'='COLD'
(Other definitions for cold that I've seen before include "Frigid" , "Common ailment" , "Unsympathetic" , "Unfriendly" , "Lacking emotion" .)