Boatman — to relish, ultimately, or detest (5)
I believe the answer is:
abhor
'detest' is the definition.
(abhorring is a kind of detesting)
'boatman to relish ultimately or' is the wordplay.
'boatman' becomes 'ab' (abbreviation for able seaman).
'to' means one lot of letters go next to another.
'ultimately' indicates one should take the final letters.
The last letter of 'relish' is 'h'.
'ab'+'h'+'or'='ABHOR'
(Other definitions for abhor that I've seen before include "Sea between Africa and Australia" , "Detest, hate" , "Can't bear" , "Not like" , "cannot stand" .)