Ointment ruined linen mitt almost entirely (8)
I believe the answer is:
liniment
'ointment' is the definition.
(thesaurus)
'ruined linen mitt almost entirely' is the wordplay.
'ruined' indicates an anagram.
'almost entirely' means to remove the last letter.
'linen'+'mitt'='linenmitt'
'linenmitt' with its last letter taken away is 'linenmit'.
'linenmit' is an anagram of 'LINIMENT'.
(Other definitions for liniment that I've seen before include "application" , "Liquid for rubbing into skin" , "Embrocation" , "Lotion" , "Medicinal liquid for the muscles" .)