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