Allow any number to ignore period of fasting (3)
I believe the answer is:
let
'allow' is the definition.
('let' can be a synonym of 'allow')
'any number to ignore period of fasting' is the wordplay.
'any number' becomes 'n' (I've seen this in other clues).
'to ignore' suggests deleting specific letters.
'period of fasting' becomes 'lent' (I've seen this before).
'lent' with 'n' removed is 'LET'.
(Other definitions for let that I've seen before include "Permit or rent" , "Permit; lease" , "Allow - hire" , "Allow the obstruction" , "Rent out: grant" .)