Add one new hint about day (7)
I believe the answer is:
include
'add' is the definition.
(including is a kind of adding)
'one new hint about day' is the wordplay.
'one new' becomes 'in' (I've seen this in other clues).
'hint' becomes 'clue' ('clue' can be a synonym of 'hint'**).
'about' indicates putting letters inside.
'day' becomes 'd'.
'clue' placed around 'd' is 'clude'.
'in'+'clude'='INCLUDE'
(Other definitions for include that I've seen before include "Consider as part of something" , "Contain as part of group" , "Take as part of a group" , "Comprise; allow to take part" , "Contain - embrace" .)