Unspecified doctrine originally inspiring so many (3)
I believe the answer is:
ism
'unspecified doctrine' is the definition.
(I've seen this before)
'originally inspiring so many' is the wordplay.
'originally' suggests taking the first letters.
The initial letters of 'inspiring so many' is 'ISM'.
(Other definitions for ism that I've seen before include "Belief system" , "Suffix denoting doctrine or practice" , "Informally a doctrine, or suffix for one" , "Ideological movement" , "Informal doctrine - any one" .)