Case of importation business being weak (6)
I believe the answer is:
infirm
'weak' is the definition.
(I know that weak can be written as infirm)
'case of importation business' is the wordplay.
'case of' means to remove the middle letters (outsides of).
'business' becomes 'firm' (firm is a kind of business).
'importation' with its centre taken out is 'in'.
'in'+'firm'='INFIRM'
'being' is the link.
(Other definitions for infirm that I've seen before include "not so healthy" , "Feeble from age" , "stable? No" , "Feeble, decrepit" , "'Weak, ill (6)'" .)