Claim thallium breaks link (5)
I believe the answer is:
title
'claim' is the definition.
(I know that claim can be written as title)
'thallium breaks link' is the wordplay.
'thallium' becomes 'Tl' (Tl is the chemical symbol for thallium).
'breaks' is an insertion indicator (inserted letters break into the word).
'link' becomes 'tie' (tying is a kind of linking).
'tl' inserted within 'tie' is 'TITLE'.
(Other definitions for title that I've seen before include "Form of address - name" , "Name of, eg, book" , "Legal possession" , "The name of a work of art or book" , "Right of ownership" .)