Hardly a mark on the outside of the case (6)
I believe the answer is:
scarce
'hardly' is the definition.
('scarce' can be a synonym of 'hardly')
'a mark on the outside of the case' is the wordplay.
'a mark' becomes 'scar' (I've seen this before).
'on the outside of' suggests removing the centre (I've seen 'on the outside' mean this).
'case' with its middle removed is 'ce'.
'scar'+'ce'='SCARCE'
(Other definitions for scarce that I've seen before include "Insufficient to meet demand" , "Like gold dust" , "Not often come across" , "Falling short of demand" , "Wanting" .)