Letters from oldest heir showing what belongs to them? (6)
I believe the answer is:
theirs
'what belongs to them?' is the definition.
'letters from oldest heir showing' is the wordplay.
'letters from' indicates a hidden word (a selection of letters from the clue).
'THEIRS' is hidden in the letters of 'oldest heir showing'.
(Other definitions for theirs that I've seen before include "Belonging to those people" , "Owned by them" .)