A thousand invested in somewhere to sleep that’s heated (5)
I believe the answer is:
baked
'heated' is the definition.
(baking is a kind of heating)
'a thousand invested in somewhere to sleep' is the wordplay.
'thousand' becomes 'k' ('K' can be a synonym of 'thousand'**).
'invested in' means one lot of letters goes inside another.
'somewhere to sleep' becomes 'bed'.
'a'+'k'='ak'
'ak' put within 'bed' is 'BAKED'.
'that's' acts as a link.
(Other definitions for baked that I've seen before include "Cooked in an oven" , "Cooked (bread/cakes)" , "Cooked like a cake" , "Got very hot in jacket" , "cold before 8 [STATE]..." .)