One northern company is in poor health (6)
I believe the answer is:
infirm
'in poor health' is the definition.
(thesaurus)
'one northern company' is the wordplay.
'one' becomes 'i' (roman numeral).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'company' becomes 'firm' (I've seen this before).
'i'+'n'+'firm'='INFIRM'
'is' is the link.
(Other definitions for infirm that I've seen before include "Not well" , "Doddery" , "unstable" , "Frail" , "Weak and feeble" .)