Like a mansion said to be damp and unhealthy (6)
I believe the answer is:
rheumy
'be damp and unhealthy' is the definition.
(I've seen this before)
'like a mansion said' is the wordplay.
'like a mansion' becomes 'roomy' (having lots of room).
'said' shows a homophone (sound like).
'roomy' sounds like 'RHEUMY'.
'to' is the link.
(Other definitions for rheumy that I've seen before include "Moist, damp, wet - especially of air" , "Catarrh-filled" , "congestion" , "Arthritic" , "Of eyes or nose, having watery discharge" .)