Last demons ruined last in convent (7)
I believe the answer is:
endmost
'last' is the definition.
(I've seen this before)
'demons ruined last in convent' is the wordplay.
'ruined' indicates an anagram.
'last in' indicates one should take the final letters.
The last letter of 'convent' is 't'.
'demons' with letters rearranged gives 'endmos'.
'endmos'+'t'='ENDMOST'
(Other definitions for endmost that I've seen before include "Most distant" , "Farthest" , "final" , "Last in a row" , "Last in the line" .)