Sword in Rome forged by Cassius once (8)
I believe the answer is:
claymore
'sword' is the definition.
(thesaurus)
'rome forged by cassius once' is the wordplay.
'forged' is an anagram indicator.
'by' says to put letters next to each other.
'cassius once' becomes 'clay' (Cassius Clay, the boxer Mohammad Ali).
'rome' with letters rearranged gives 'more'.
'more' after 'clay' is 'CLAYMORE'.
'in' is the link.
(Other definitions for claymore that I've seen before include "Scottish two-edged sword" , "pipe" , "Scottish sword wielded by my oracle" , "Scottish Highlanders' broadsword" , "Two-edged broadsword" .)