Accustom one to being almost ruined, perhaps (5)
I believe the answer is:
inure
'accustom' is the definition.
(I know that accustom is a is a more specific form of the action of inure)
'one to being almost ruined perhaps' is the wordplay.
'one to being almost' means to remove the last letter (most but not all of the letters).
'perhaps' indicates anagramming the letters.
'ruined' with its final letter removed is 'ruine'.
'ruine' with letters rearranged gives 'INURE'.
(Other definitions for inure that I've seen before include "Harden or accustom as to something unpleasant" , "Come to accept" , "Harden, make accustomed to" , "Accustom (someone) to something unpleasant" , "Harden or season" .)