Incurable as warhorse cut in middle of winter (10)
I believe the answer is:
inveterate
'incurable' is the definition.
(thesaurus)
'warhorse cut in middle of winter' is the wordplay.
'warhorse' becomes 'veteran' ('veteran' can be a synonym of 'warhorse').
'cut' means to remove the last letter.
'in' indicates putting letters inside.
'middle of' indicates the central letters.
The middle of 'winter' is 'inte'.
'veteran' with its final letter removed is 'vetera'.
'vetera' put inside 'inte' is 'INVETERATE'.
'as' acts as a link.
(Other definitions for inveterate that I've seen before include "Long-formed (bad habit)" , "Long-established" , "Habitual" , "Hardened" , "Firmly established by usage or custom" .)