You'll need a bottle opener for one (it's the hard stuff) (4)
I believe the answer is:
bone
'stuff' is the definition.
(bone is a kind of stuff)
'you'll need a bottle opener for one it's the hard' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'one' and 'one' is found within the answer.
A single letter 'b' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bone that I've seen before include "Eg, rib" , "See 6" , "Scapula, radius or tibia?" , "Possibly hammer" , "#Close to it is risque" .)