Always right to follow the day before Christmas (4)
I believe the answer is:
ever
'always' is the definition.
('ever' can be a synonym of 'always')
'right to follow the day before christmas' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to follow' means one lot of letters go next to another.
'the day before christmas' becomes 'eve' (I've seen this before).
'r' put after 'eve' is 'EVER'.
(Other definitions for ever that I've seen before include "constantly" , "Always and permanent" , "At all times, always" , "Perpetually" , "Continually" .)