Save family mansion? (9)
I believe the answer is:
stockpile
'save' is the definition.
(I've seen this before)
'family mansion?' is the wordplay.
'family' becomes 'stock'.
'mansion?' becomes 'pile' (pile can mean a large building).
'stock'+'pile'='STOCKPILE'
(Other definitions for stockpile that I've seen before include "Save" , "Accumulate reserves" , "Accumulated goods" , "Gather" , "Amass" .)