Put a leper through treatment that's going on for ever (9)
I believe the answer is:
perpetual
'going on for ever' is the definition.
(I've seen this before)
'put a leper through treatment' is the wordplay.
'through treatment' indicates anagramming the letters (letters are treated or processed).
'put'+'a'+'leper'='putaleper'
'putaleper' with letters rearranged gives 'PERPETUAL'.
'that's' is the link.
(Other definitions for perpetual that I've seen before include "Endless, lasting" , "Going on for ever" , "Continuous - permanent" , "Continuing forever or indefinitely" , "Constant" .)