Give up, imprisoned by entirely heartless justice (6)
I believe the answer is:
equity
'justice' is the definition.
(I know that equity is a type of justice)
'give up imprisoned by entirely heartless' is the wordplay.
'give up' becomes 'quit' (both can mean to resign).
'imprisoned by' indicates putting letters inside.
'heartless' suggests removing the centre.
'entirely' with its centre removed is 'ey'.
'quit' put inside 'ey' is 'EQUITY'.
(Other definitions for equity that I've seen before include "Even-handedness" , "Group of performers" , "Fairness for actors' trade union" , "Fairness, impartial treatment" , "The ownership interest of shareholders in a company" .)