The doubting Apostle (6)
I believe the answer is:
thomas
'the doubting apostle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for thomas that I've seen before include "Dylan ..., ... Aquinas" , "He had doubts" , "Boy's name - sceptic" , "-- Moore, Irish poet" , "steam engine" .)