Boatman — to relish, ultimately, or detest (5)
I believe the answer is:
Here is my best explanation:
'detest' is the definition.
(abhorring is a kind of detesting)
'boatman to relish ultimately or' is the subsidiary indication.
'boatman' becomes 'ab' (abbreviation for able seaman).
'to' means one lot of letters go next to another.
'ultimately' suggests the final letters.
The final letter of 'relish' is 'h'.
(Other definitions for abhor that I've seen before include "can't stand", "Not like", "reject", "Sea between Africa and Australia", "Loathe, detest", "Regard with disgust and hatred", "Dislike very much", "Can't stand", "Detest, hate" and "Recoil from".)