Doubting apostle (6)
I believe the answer is:
thomas
'doubting apostle' is the definition.
(I've seen this before)
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" .)