One hundred per cent with Christmas decoration (6)
I believe the answer is:
wholly
'one hundred per cent' is the definition.
(I've seen this before)
'with christmas decoration' is the wordplay.
'with' becomes 'w' (abbreviation).
'christmas decoration' becomes 'holly' (I've seen this in another clue).
'w'+'holly'='WHOLLY'
(Other definitions for wholly that I've seen before include "Altogether" , "in full?" , "Entirely, fully" , "One hundred per cent" , "Totally" .)