One doubting sign coat needs replacing (8)
I believe the answer is:
agnostic
'one doubting' is the definition.
(someone unsure on the topic of religion)
'sign coat needs replacing' is the wordplay.
'needs replacing' indicates anagramming the letters (letters placed in new positions).
'sign'+'coat'='signcoat'
'signcoat' is an anagram of 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "One believing we can know nothing of the existence of any god" , "Believing that nothing can be known about the existence of God" , "Person who thinks nothing can be known about God" , "I doubt" , "Coasting (anag)" .)