Sailor with zero time, competent but loathsome (10)
I believe the answer is:
abominable
'loathsome' is the definition.
(I've seen this in another clue)
'sailor with zero time competent' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'with' is a charade indicator (letters next to each other).
'zero' becomes 'O' (O resembles 0).
'time' becomes 'min' (short for minute).
'competent' becomes 'able' (similar in meaning).
'ab'+'o'+'min'+'able'='ABOMINABLE'
'but' acts as a link.
(Other definitions for abominable that I've seen before include "Hateful" , "Causing moral revulsion" , "Very bad, terrible" , "Atrocious" , "Odious" .)