Get used to terrible ruin by the end (5)
I believe the answer is:
inure
'get used to' is the definition.
(I've seen this in another clue)
'terrible ruin by the end' is the wordplay.
'terrible' is an anagram indicator.
'by' says to put letters next to each other.
'end' says to take the final letters.
The final letter of 'the' is 'e'.
'ruin' anagrammed gives 'inur'.
'inur'+'e'='INURE'
(Other definitions for inure that I've seen before include "Temper, toughen" , "get tougher" , "Come to accept" , "Harden or accustom as to something unpleasant" , "Accustom (someone) to something unpleasant" .)