After exam success I have to become lethargic (7)
I believe the answer is:
passive
'lethargic' is the definition.
(thesaurus)
'after exam success i have' is the wordplay.
'after' is a charade indicator (letters next to each other).
'exam success' becomes 'pass' (I've seen this before).
'i have' becomes 'ive'.
'pass'+'ive' is 'PASSIVE'.
'to become' acts as a link.
(Other definitions for passive that I've seen before include "Submissive" , "Not actively responsive" , "Slow to react" , "Unreactive" , "Inactive or non-resistant" .)