Doubting apostle (6)
I believe the answer is:
thomas
'doubting apostle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for thomas that I've seen before include "One of the Twelve Apostles" , "Famous tank engine" , "The Apostle who initially doubted the Resurrection" , "He had doubts" , "He needs convincing" .)