Promising to hold retreat, initially after a series of prayers (6)
I believe the answer is:
rosary
'series of prayers' is the definition.
(I've seen this before)
'promising to hold retreat initially after a' is the wordplay.
'promising' becomes 'rosy' (synonyms).
'to hold' is an insertion indicator.
'initially' says to take the initial letters.
'after' says to put letters next to each other.
The initial letter of 'retreat' is 'r'.
'r' after 'a' is 'ar'.
'rosy' placed around 'ar' is 'ROSARY'.
(Other definitions for rosary that I've seen before include "Series of prayers" , "Flower garden" , "RC beads" , "Prayer aid" , "Catholic prayer beads" .)