At home a gym is unsuitable (5)
I believe the answer is:
inapt
'unsuitable' is the definition.
(I've seen this in another clue)
'at home a gym' is the wordplay.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'gym' becomes 'PT' (Physical Training).
'in'+'a'+'pt'='INAPT'
'is' is the link.
(Other definitions for inapt that I've seen before include "It's clumsy" , "Useless" , "Not suitable or appropriate" , "Out of place" , "not seemly" .)