One all in purple initially summoned church guards (5,5)
I believe the answer is:
spent force
'one all in' is the definition.
(someone 'all in' is exhausted)
'purple initially summoned church guards' is the wordplay.
'initially' says to take the initial letters.
'summoned' becomes 'sent for'.
'church' becomes 'CE' (abbreviation for Church of England).
'guards' is an insertion indicator (inserted letters are guarded).
The first letter of 'purple' is 'p'.
'sentfor'+'ce'='sentforce'
'p' placed inside 'sentforce' is 'SPENT FORCE'.
(Other definitions for spent force that I've seen before include "Has-been" , "power exhausted" .)