One stirring a private army to resistance (8)
I believe the answer is:
agitator
'one stirring' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a private army to resistance' is the wordplay.
'private' becomes 'GI' (GI can informally mean a US soldier).
'army' becomes 'TA' (abbreviation for Territorial Army).
'resistance' becomes 'r' (electronics abbreviation).
'a'+'gi'+'ta'+'to'+'r'='AGITATOR'
Can you help me to learn more?
(Other definitions for agitator that I've seen before include "washing machine" , "One fomenting trouble" , "Got tiara for a trouble-maker" , "Person stirring up public feeling" , "One causing trouble" .)