Mariner prophet keeps at a great distance (8)
I believe the answer is:
seafarer
'mariner' is the definition.
(I know that mariner can be written as seafarer)
'prophet keeps at a great distance' is the wordplay.
'prophet' becomes 'seer' (both can mean someone who tells the future).
'keeps' is an insertion indicator (inserted letters are kept inside).
'at a great distance' becomes 'afar' (I've seen this in another clue).
'seer' enclosing 'afar' is 'SEAFARER'.
(Other definitions for seafarer that I've seen before include "A traveller on the ocean, like sailor" , "Salt" , "Traveller by ship" , "One going by ship" , "man being attracted by waves?" .)