Substantial weight put on a religious ruling (5)
I believe the answer is:
fatwa
'religious ruling' is the definition.
(I've seen this before)
'substantial weight put on a' is the wordplay.
'substantial' becomes 'fat' (I've seen this before).
'weight' becomes 'w'.
'put on' is a charade indicator (letters next to each other).
'fat'+'w'+'a'='FATWA'
(Other definitions for fatwa that I've seen before include "legal opinion" , "Legal ruling by Islamic religious leader" , "Islamic opinion" , "Ruling by Islamic religious leader, say Ayatollah" , "Ruling by Islamic leader, as on Salmon Rushdie" .)