Regularly ahead of time by end of day (6)
I believe the answer is:
yearly
'regularly' is the definition.
(thesaurus)
'ahead of time by end of day' is the wordplay.
'ahead of time' becomes 'early' (synonyms).
'by' says to put letters next to each other.
'end of' suggests the final letters.
The last letter of 'day' is 'y'.
'early' after 'y' is 'YEARLY'.
(Other definitions for yearly that I've seen before include "Happening every twelve months" , "Every tenth of a decade" , "At annual intervals" , "in each revolution?" , "Unpleasant noise" .)