At home adult takes exercise, being unfit (5)
I believe the answer is:
inapt
'unfit' is the definition.
(thesaurus)
'at home adult takes exercise' is the wordplay.
'at home' becomes 'in' (similar in meaning).
'adult' becomes 'a' (abbreviation).
'takes' means one lot of letters go next to another.
'exercise' becomes 'PT' (Physical Training).
'in'+'a'+'pt'='INAPT'
'being' is the link.
(Other definitions for inapt that I've seen before include "Not fit, unsuitable" , "Unsuited" , "Not suitable or appropriate" , "not seemly" , "It's clumsy" .)