Tendon; gristle (5)
I believe the answer is:
sinew
This clue is a double definition.
'tendon' is the first definition.
(I know that tendon can be written as sinew)
'gristle' is the second definition.
(I've seen this before)
(Other definitions for sinew that I've seen before include "Gristle" , "fibre" , "Tough connective tissue" , "source of physical strength" , "Tendon, for example" .)