The resentment an heiress can show (3)
I believe the answer is:
ire
'the resentment' is the definition.
(I've seen this in another clue)
'an heiress can show' is the wordplay.
'show' indicates the answer is hidden within the clue.
'IRE' can be found hidden inside 'heiress can'.
(Other definitions for ire that I've seen before include "Anger, rage" , "Irritation, anger" , "Displeasure" , "passion" , "Intense anger" .)