Frail, but part of the business? (6)
I believe the answer is:
infirm
'frail' is the definition.
(similar in meaning)
'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'
'but' is the link.
(Other definitions for infirm that I've seen before include "Feeble, decrepit" , "Not in good health" , "stable? No" , "Doddery" , "unstable" .)