Wicked nuns stripped in front of Christmas decoration -- heartless! (6)
I believe the answer is:
unholy
'wicked' is the definition.
(I know that wicked can be written as unholy)
'nuns stripped in front of christmas decoration heartless' is the wordplay.
'stripped' indicates the central letters (outside letters removed).
'in front of' means one lot of letters go next to another.
'christmas decoration' becomes 'holly' (I've seen this in another clue).
'heartless' means to remove the middle letters.
The middle of 'nuns' is 'un'.
'holly' with its centre taken out is 'holy'.
'un'+'holy'='UNHOLY'
(Other definitions for unholy that I've seen before include "Sinful, wicked" , "Base" , "Hellish" , "Depraved" , "Not sacred; outrageous" .)