I'm not sure exercise even is for many (7)
I believe the answer is:
umpteen
'for many' is the definition.
(similar in meaning)
'i'm not sure exercise even' is the wordplay.
'i'm not sure' becomes 'um' (expression of hesitation).
'exercise' becomes 'PT' (Physical Training).
'even' becomes 'e'en' (synonyms).
'um'+'pt'+'een'='UMPTEEN'
'is' acts as a link.
(Other definitions for umpteen that I've seen before include "Unspecified large number" , "Informally, very many" , "Lots of things on pet menu" , "Plentiful" , "Indefinitely many" .)