Affluent heiresses, in reduced circumstances, becoming possessive (5)
I believe the answer is:
their
'possessive' is the definition.
(I've seen this before)
'affluent heiresses in reduced circumstances' is the wordplay.
'in reduced circumstances' says the answer is hidden in the clue.
'THEIR' can be found hidden inside 'affluent heiresses'.
'becoming' is the link.
(Other definitions for their that I've seen before include "Belonging to those people" , "Relating to them" , "Those people's" , "Possessive pronoun" , "Belonging to him (perhaps wrongly)" .)