French are blocking the present item in argument (6)
I believe the answer is:
thesis
'argument' is the definition.
(thesaurus)
'french are blocking the present item' is the wordplay.
'french are' becomes 'es' ('are' in French).
'blocking' is an insertion indicator.
'the present item' becomes 'this'.
'es' inserted within 'this' is 'THESIS'.
'in' is the link.
(Other definitions for thesis that I've seen before include "View advanced for purposes of argument" , "Article by son is in paper" , "Researched dissertation" , "Learned dissertation" , "Dissertation presented for a doctorate" .)