Historic old chapel in ruins (7)
I believe the answer is:
epochal
'historic' is the definition.
(I've seen this before)
'old chapel in ruins' is the wordplay.
'old' becomes 'o' (abbreviation).
'in ruins' is an anagram indicator.
'o'+'chapel'='ochapel'
'ochapel' with letters rearranged gives 'EPOCHAL'.
(Other definitions for epochal that I've seen before include "Historic" , "of a certain age" , "of a particular age" , "Of a period in history" , "beginning a new era" .)