Small piece of furniture, firmly fixed (6)
I believe the answer is:
stable
'firmly fixed' is the definition.
(I've seen this before)
'small piece of furniture' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'piece of furniture' becomes 'table' (table is a kind of piece of furniture).
's'+'table'='STABLE'
(Other definitions for stable that I've seen before include "farm building" , "'Steady, not given to sudden change (6)'" , "Maintaining equilibrium" , "Equine housing" , "Housing for a horse" .)