Sour, hard-hearted, fearsome fellow (6)
I believe the answer is:
tartar
'fearsome fellow' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'sour hard-hearted' is the wordplay.
'sour' becomes 'tart' (similar in meaning).
'hearted' means to look at the middle letters.
The middle of 'hard' is 'ar'.
'tart'+'ar'='TARTAR'
Can you help me to learn more?
(Other definitions for tartar that I've seen before include "Fearsome person: kind of sauce" , "Incrustation on teeth and gums" , "plaque" , "Cream of - - : baking ingredient" , "This sauce often goes with fish" .)