A toddy drunk certainly makes you regular (3-2-3)
I believe the answer is:
day-to-day
'regular' is the definition.
('day-to-day' can be similar in meaning to 'regular')
'a toddy drunk certainly' is the wordplay.
'drunk' is an anagram indicator.
'certainly' becomes 'ay' (variant spelling of 'aye').
'a'+'toddy'='atoddy'
'atoddy' anagrammed gives 'daytod'.
'daytod'+'ay'='DAY-TO-DAY'
'makes you' is the link.
(Other definitions for day-to-day that I've seen before include "Mundane, routine" , "Normal" .)