Outside one is flogged hard (5)
I believe the answer is:
solid
'hard' is the definition.
('solid' can be similar in meaning to 'hard')
'outside one is flogged' is the wordplay.
'outside' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'is flogged' becomes 'sold' (I've seen this before).
'i' inserted within 'sold' is 'SOLID'.
(Other definitions for solid that I've seen before include "Firm and stable" , "Concrete or reilable" , "Of substance" , "Hard and compact" , "Neither liquid nor gas" .)