Detests unseemly haste (5)
I believe the answer is:
hates
'detests' is the definition.
('hate' can be a synonym of 'detest')
'unseemly haste' is the wordplay.
'unseemly' indicates anagramming the letters.
'haste' with letters rearranged gives 'HATES'.
(Other definitions for hates that I've seen before include "passionate feelings" , "can't stand" , "Abhors" , "Dislikes intensely" , "Detests" .)