Arrange that I have a posse formed on the day (7)
I believe the answer is:
dispose
'arrange' is the definition.
(I've seen this before)
'i have a posse formed on the day' is the wordplay.
'have' is a charade indicator (letters next to each other).
'formed' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'the day' becomes 'd' (abbreviation for day).
'posse' is an anagram of 'spose'.
'i'+'spose' is 'ispose'.
'ispose' put after 'd' is 'DISPOSE'.
'that' acts as a link.
(Other definitions for dispose that I've seen before include "Arrange - to get rid (of)" , "Incline" , "Place in order (as God does)" , "Arrange (in order)" , "make an arrangement" .)