Duty makes heart get hard (5)
I believe the answer is:
chore
'duty' is the definition.
(I know that chore is a type of duty)
'heart get hard' is the wordplay.
'heart' becomes 'core' (synonyms).
'get' indicates putting letters inside.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'core' enclosing 'h' is 'CHORE'.
'makes' acts as a link.
(Other definitions for chore that I've seen before include "Boring job" , "Household job" , "Tedious routine task" , "Manipulate; name" , "easy option? Not so!" .)