The Lord has one for each letting in light (6)
I believe the answer is:
prayer
'the lord has one' is the definition.
(the Lord's Prayer)
'each letting in light' is the wordplay.
'each' becomes 'per' ('per' can mean for each).
'letting in' means one lot of letters goes inside another.
'light' becomes 'ray' (ray is a kind of light).
'per' enclosing 'ray' is 'PRAYER'.
'for' acts as a link.
(Other definitions for prayer that I've seen before include "Request (maybe to God)" , "Litany" , "Small chance" , "Collect - petition" , "Words to a deity" .)