One house of a number in ruin! (5)
I believe the answer is:
manor
'one house' is the definition.
(manor is a kind of house)
'number in ruin' is the wordplay.
'number' becomes 'no' (abbreviation).
'in' indicates putting letters inside.
'ruin' becomes 'mar' (I've seen this before).
'no' going within 'mar' is 'MANOR'.
'of a' acts as a link.
(Other definitions for manor that I've seen before include "Operational base" , "house on estate" , "Large country house with lands" , "patch" , "Roman (anag.)" .)