Hooked cadet did badly (8)
I believe the answer is:
addicted
'hooked' is the definition.
('addict' can be a synonym of 'hook')
'cadet did badly' is the wordplay.
'badly' indicates anagramming the letters.
'cadet'+'did'='cadetdid'
'cadetdid' with letters rearranged gives 'ADDICTED'.
(Other definitions for addicted that I've seen before include "Hooked compulsively as by drugs" , "Drug dependent" , "Cadet did become compelled to use harmful substance (8)" , "Dependent on drugs" , "Unable to stop" .)