A small firm demonstrates something of value (5)
I believe the answer is:
asset
'something of value' is the definition.
(I've seen this before)
'a small firm' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'firm' becomes 'set' (both can mean fixed).
'a'+'s'+'set'='ASSET'
'demonstrates' acts as a link.
(Other definitions for asset that I've seen before include "valuable sort" , "blessing" , "Advantage, strength" , "Item of value or advantage" , "Something useful" .)