Firm one sold out (5)
I believe the answer is:
solid
'firm' is the definition.
(I know that firm can be written as solid)
'one sold out' is the wordplay.
'one' becomes 'i' (Roman numeral).
'out' indicates putting letters inside.
'i' placed within 'sold' is 'SOLID'.
(Other definitions for solid that I've seen before include "Firmly constructed" , "Firm and stable" , "Firm, strong" , "Neither liquid nor gas" , "Weighty" .)