Famous tank engine (6)
I believe the answer is:
thomas
'famous tank engine' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for thomas that I've seen before include "Ancient writing instruments" , "Dylan --; -- Edison" , "Doubter - tank engine" , "The Blue Boy painter, d. 1788" , "Dylan ..., ... Aquinas" .)