Wise man in boat, heading off (6)
I believe the answer is:
oracle
'wise man' is the definition.
(I've seen this before)
'boat heading off' is the wordplay.
'boat' becomes 'coracle' (coracle is a kind of boat).
'heading off' means to remove the first letter.
'coracle' with its initial letter taken away is 'ORACLE'.
'in' is the link.
(Other definitions for oracle that I've seen before include "Someone of great wisdom" , "Prophet or sage" , "Augury" , "(Prophecy given at) shrine" , "Person of great wisdom, as in Delphi of old" .)