One good toilet in blockhouse (5)
I believe the answer is:
igloo
'blockhouse' is the definition.
(house made of ice blocks)
'one good toilet' is the wordplay.
'one' becomes 'i' (Roman numeral).
'good' becomes 'g' (abbreviation).
'toilet' becomes 'loo' (loo can informally mean a toilet).
'i'+'g'+'loo'='IGLOO'
'in' acts as a link.
(Other definitions for igloo that I've seen before include "Ice-block construction" , "Dome-shaped dwelling" , "Inuit hut" , "Inuit dwelling" , "Hut built of snow" .)