When winter comes, sooner than expected, after end of January (6)
I believe the answer is:
yearly
'when winter comes' is the definition.
'sooner than expected after end of january' is the wordplay.
'sooner than expected' becomes 'early' ('early' can be a synonym of 'sooner than expected').
'after' says to put letters next to each other.
'end of' says to take the final letters.
The final letter of 'january' is 'y'.
'early' put after 'y' is 'YEARLY'.
(Other definitions for yearly that I've seen before include "on a regular basis" , "Happening every twelve months" , "in each revolution?" , "when Christmas comes round" , "Annually" .)