Endlessly regretting introduction to such decayed buildings (5)
I believe the answer is:
ruins
'decayed buildings' is the definition.
(remains of buildings)
'endlessly regretting introduction to such' is the wordplay.
'endlessly' means to remove the last letter.
'regretting' becomes 'ruing' ('rue' can be a synonym of 'regret').
'introduction to' says to take the initial letters.
The first letter of 'such' is 's'.
'ruing' with its final letter taken off is 'ruin'.
'ruin'+'s'='RUINS'
(Other definitions for ruins that I've seen before include "Damages beyond repair" , "Wrecked buildings" , "Spoils" , "Mars" , "Derelict remains of building" .)