Forgetful man exercising energy thus (7)
I believe the answer is:
amnesic
'forgetful' is the definition.
(similar in meaning)
'man exercising energy thus' is the wordplay.
'exercising' is an anagram indicator (letters exercise or move around).
'energy' becomes 'e' (physics symbol as in E=mc2).
'thus' becomes 'sic' ('thus' in Latin).
'man' with letters rearranged gives 'amn'.
'amn'+'e'+'sic'='AMNESIC'
(Other definitions for amnesic that I've seen before include "Forgetting" , "not showing Total Recall?" , "Forgetful" , "Likely to forget" , "Suffering total or partial memory loss" .)