Fierce fellows who hopelessly start to hide their hard heart (7)
I believe the answer is:
tartars
'their hard heart' is the definition.
I can't tell whether this defines the answer.
'fierce fellows who hopelessly start to hide' is the wordplay.
'fierce fellows who' becomes 'ar' (I can't justify this - if you can you should believe this answer much more).
'hopelessly' indicates an anagram.
'to hide' means one lot of letters goes inside another.
'start' is an anagram of 'ttars'.
'ar' put within 'ttars' is 'TARTARS'.
Can you help me to learn more?
(Other definitions for tartars that I've seen before include "Warlike Mongols" , "Rat arts are ferocious and formidable people" .)