Ben on his broken tibia (8)
I believe the answer is:
shinbone
'tibia' is the definition.
(synonyms)
'ben on his broken' is the wordplay.
'broken' is an anagram indicator.
'ben'+'on'+'his'='benonhis'
'benonhis' is an anagram of 'SHINBONE'.
(Other definitions for shinbone that I've seen before include "The less formal tibia" , "It is also called the tibia" , "that supports member" , "bit of leg" .)