Allowed knight to leave fast (3)
I believe the answer is:
let
'allowed' is the definition.
(to let is to allow or permit)
'knight to leave fast' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'to leave' suggests deleting specific letters.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'lent' with 'n' removed is 'LET'.
(Other definitions for let that I've seen before include "Allow on the tennis court" , "inefficient service" , "Allow, don't hinder" , "& 15 Triple (anag.)" , "Permit or rent" .)