Off and on, this day is fairly good (4)
I believe the answer is:
tidy
'fairly good' is the definition.
'off and on this day' is the wordplay.
'off and on' indicates alternate letters (letters used at regular intervals).
'this'+'day'='thisday'
The alternate letters of 'thisday' are 'TIDY'.
'is' acts as a link.
(Other definitions for tidy that I've seen before include "Considerable (sum); neat" , "Systematic; substantial" , "Neaten" , "so substantial a sum" , "Set in order" .)