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'm unsure of the 'the hard' bit.
'grit' is the second definition.
(thesaurus)
'is' acts as a 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" .)