Housing is excessive in prison (7)
I believe the answer is:
cottage
'housing' is the definition.
(I know that cottage is a type of house)
'excessive in prison' is the wordplay.
'excessive' becomes 'OTT' (short for 'over the top').
'in' indicates putting letters inside.
'prison' becomes 'cage' (I've seen this before).
'ott' put inside 'cage' is 'COTTAGE'.
'is' is the link.
(Other definitions for cottage that I've seen before include "Small place" , "building" , "Country dwelling" , "Small home" , "Small living space" .)