Very unhealthy, like large country houses (6)
I believe the answer is:
villas
'large country houses' is the definition.
(type of large residence)
'very unhealthy like' is the wordplay.
'very' becomes 'v' (abbreviation).
'unhealthy' becomes 'i'll' (associated in meaning).
'like' becomes 'as'.
'v'+'ill'+'as'='VILLAS'
(Other definitions for villas that I've seen before include "Large houses or holiday homes abroad" , "country houses" .)