Firm I sold out (5)

I believe the answer is:

solid

'firm' is the definition.
(I know that firm can be written as solid)

'i sold out' is the wordplay.
'out' indicates putting letters inside.
'i' put into 'sold' is 'SOLID'.

(Other definitions for solid that I've seen before include "Neither liquid nor gas" , "continuous" , "Reliable; not hollow" , "Firm - substantial" , "Idols (anag.)" .)

I've seen this clue in The Globe and Mail.

Want a hint initially instead of a full solution? Install my app