Left in "pious," it being in keeping with Christmas (5)
I believe the answer is:
holly
'it being in keeping with christmas' is the definition.
The definition suggests a singular noun which matches the answer.
'left in pious' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'pious' becomes 'holy' (I've seen this in another clue).
'l' going within 'holy' is 'HOLLY'.
Can you help me to learn more?
(Other definitions for holly that I've seen before include "Ilex -- popular at Christmas" , "Evergreen tree associated with Christmas" , "Evergreen tree with spiky leaves" , "... and ivy, Yuletide decoration" , "Prickly-leaved shrub" .)