After the start, rowing boat is a source of revelation (6)
I believe the answer is:
oracle
'a source of revelation' is the definition.
(oracle can mean a source of mystical revelations)
'after the start rowing boat' is the wordplay.
'after the start' means to remove the first letter.
'rowing boat' becomes 'coracle' (I've seen this before).
'coracle' with its initial letter taken away is 'ORACLE'.
'is' acts as a link.
(Other definitions for oracle that I've seen before include "Medium for prophecy" , "Augury" , "Holy place in ancient times where gods prophesied" , "Source of prophecies" , "sign of things to come" .)