One regulation I'd set up for religious festival (6)
I believe the answer is:
diwali
'religious festival' is the definition.
(I've seen this before)
'one regulation i'd set up' is the wordplay.
'one' becomes 'i' (Roman numeral).
'regulation' becomes 'law' (I've seen this before).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'i'+'law'+'id'='ilawid'
'ilawid' reversed gives 'DIWALI'.
'for' acts as a link.
(Other definitions for diwali that I've seen before include "Indian festival" , "Festival in honour of Lakshmi" , "Hindu festival; I'd wail (anag.)" , "religious festival" , "Hindu festival of lights" .)