It's hard when one is sold out (5)
I believe the answer is:
solid
'it's hard' is the definition.
(similar in meaning)
'one is sold out' is the wordplay.
'one' becomes 'i' (Roman numeral).
'out' is an insertion indicator.
'i' inserted into 'sold' is 'SOLID'.
'when' is the link.
(Other definitions for solid that I've seen before include "Three-dimensional shape, not hollow" , "Not hollow or liquid" , "Strongly made or built" , "Firm - sound - reliable" , "Dense" .)