One who doesn't entirely believe in initially culling goats, confounded (8)
I believe the answer is:
agnostic
'one who doesn't entirely believe' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'in initially culling goats confounded' is the wordplay.
'initially' suggests taking the first letters.
'confounded' indicates an anagram.
The first letter of 'culling' is 'c'.
'in'+'c'+'goats'='incgoats'
'incgoats' is an anagram of 'AGNOSTIC'.
Can you help me to learn more?
(Other definitions for agnostic that I've seen before include "Person who thinks nothing can be known about God" , "One who is undecided" , "doubtful, I think" , "Uncertain type" , "Coasting with unbeliever" .)