A place for firm assumptions (8)
I believe the answer is:
premises
This could be a double definition.
'a place' is the first definition.
(premises is a kind of place)
'firm assumptions' is the second definition.
'premises' can be an answer for 'assumptions' ('premise' can be a synonym of 'assumption'). I'm not certain of the 'firm' bit.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for premises that I've seen before include "Property" , "Building, place of business" , "Places of business" , "Assumptions" , "Business buildings" .)