Sceptic clearly heartless and not quite nice (5)
I believe the answer is:
cynic
'sceptic' is the definition.
(I've seen this before)
'clearly heartless and not quite nice' is the wordplay.
'heartless' suggests removing the centre.
'and' says to put letters next to each other.
'not quite' means to remove the last letter (most but not all of the word).
'clearly' with its middle taken out is 'cy'.
'nice' with its final letter removed is 'nic'.
'cy'+'nic'='CYNIC'
(Other definitions for cynic that I've seen before include "One who believes the worst of people" , "One believing all are motivated by self-interest" , "Fault-finder" , "One thinking the worst" , "one with little faith in human nature" .)