Not fit to be part of the business (6)
I believe the answer is:
infirm
'not fit' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?
'part of the business' is the wordplay.
'part of' becomes 'in' (being in something means being part of it).
'the business' becomes 'firm' (firm is a kind of business).
'in'+'firm'='INFIRM'
'to be' is the link.
Can you help me to learn more?
(Other definitions for infirm that I've seen before include "Feeble, decrepit" , "'Weak, ill (6)'" , "Sickly" , "Weak and feeble" , "Not in good health" .)