Ailing popular company (6)
I believe the answer is:
infirm
'ailing' is the definition.
(thesaurus)
'popular company' is the wordplay.
'popular' becomes 'in'.
'company' becomes 'firm' (I've seen this before).
'in'+'firm'='INFIRM'
(Other definitions for infirm that I've seen before include "Debilitated" , "Frail" , "not so healthy" , "Feeble, decrepit" , "stable? No" .)