Cartilage in right leg: it's damaged (7)
I believe the answer is:
gristle
'cartilage' is the definition.
(synonyms)
'right leg it's damaged' is the wordplay.
'right' becomes 'r' (common abbreviation).
'damaged' indicates anagramming the letters.
'r'+'leg'+'its'='rlegits'
'rlegits' anagrammed gives 'GRISTLE'.
'in' is the link.
(Other definitions for gristle that I've seen before include "Sinew" , "Cartilage in meat" , "Tough flexible tissue" , "Stir leg to get tough tissue in meat" , "Glister (anag)" .)