Tally day’s order (7)
I believe the answer is:
summons
'order' is the definition.
(summons is a kind of order)
'tally day's' is the wordplay.
'tally' becomes 'sum' (synonyms).
'day's' becomes 'mon's' (abbreviation for Monday).
'sum'+'mons'='SUMMONS'
(Other definitions for summons that I've seen before include "Call to join" , "Calls to appear" , "Order to attend court" , "Call (to court)" , "Subpoena" .)