Hard heart flips keeping one bold (6)
I believe the answer is:
heroic
'bold' is the definition.
(similar in meaning)
'hard heart flips keeping one' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'heart' becomes 'core' (synonyms).
'flips' says the letters should be written backwards.
'keeping' is an insertion indicator (some letters kept inside).
'one' becomes 'i' (Roman numeral).
'core' in reverse letter order is 'eroc'.
'eroc' enclosing 'i' is 'eroic'.
'h'+'eroic'='HEROIC'
(Other definitions for heroic that I've seen before include "Valiant, courageous" , "Showing no fear in the face of extreme difficulty" , "Supremely courageous and brave" , "Courageous - daring" , "Dramatic" .)