Not so old housing collapsed after year (8)
I believe the answer is:
youngish
'not so old' is the definition.
'housing collapsed after year' is the wordplay.
'collapsed' is an anagram indicator.
'after' is a charade indicator (letters next to each other).
'year' becomes 'y'.
'housing' with letters rearranged gives 'oungish'.
'oungish' after 'y' is 'YOUNGISH'.
(Other definitions for youngish that I've seen before include "Rather immature" , "Not very old" , "Fairly youthful" .)