Stitch holding in tendon (5)
I believe the answer is:
sinew
'tendon' is the definition.
('sinew' can be a synonym of 'tendon')
'stitch holding in' is the wordplay.
'stitch' becomes 'sew' ('sew' can be a synonym of 'stitch').
'holding' is an insertion indicator.
'sew' placed around 'in' is 'SINEW'.
(Other definitions for sinew that I've seen before include "Force" , "Inelastic tissue" , "As synonym for 'strength'" , "Muscular strength" , "Tendon, for example" .)