The hard stuff is grit (6)
I believe the answer is:
spirit
This could be a double definition.
'the hard stuff' is the first definition.
'spirit' can be an answer for 'stuff' (thesaurus). I am not sure about the 'the hard' bit.
'grit' is the second definition.
(thesaurus)
'is' is the link.
Can you help me to learn more?
(Other definitions for spirit that I've seen before include "Vital force - apparition" , "Possibly a rum" , "Demon - drink" , "Eg, rum" , "Enthusiastic ghost" .)