Heartless guy, pinching bottom, makes one cross (6)
I believe the answer is:
Here is my best explanation:
'one cross' is the definition.
'heartless guy pinching bottom' is the subsidiary indication.
'heartless' suggests removing the centre.
'pinching' is an insertion indicator (taking inside).
'bottom' becomes 'rump' ('rump' can be a synonym of 'bottom'**).
'guy' with its centre removed is 'gy'.
'gy' placed around 'rump' is 'grumpy'.
'makes' is the link.
(Other definitions for grumpy that I've seen before include "Bad-tempered and out of sorts, like one of the Seven Dwarfs", "Cantankerous", "Ill-tempered and crabby", "likely to snap?", "Doc's irritable pal", "An ill-tempered dwarf of group of Seven", "In a bad mood", "Crabby and grouchy", "Cross" and "The cantankerous Dwarf".)