A king's taken in by charlatan's religious retreat (6)
I believe the answer is:
Here is my best explanation:
'religious retreat' is the definition.
(I've seen this before)
'a king's taken in by charlatan's' is the subsidiary indication.
'king' becomes 'r' (r is abbreviation for rex).
'taken in by' indicates putting letters inside.
'charlatan' becomes 'sham'.
'r' placed into 'sham' is 'shram'.
(Other definitions for ashram that I've seen before include "holy man's retreat", "Hindu place of religious learning located in a marsh", "Place of religious retreat for Hindus", "etreat", "Religious retreat (in India)", "Commune", "Hindu hermitage", "Indian hermitage", "Place of Hindu retreat" and "Religious community".)