Synthetic material you'll find in many longbows (5)
I believe the answer is:
nylon
'synthetic material' is the definition.
(I've seen this before)
'in many longbows' is the wordplay.
'in' says the answer is hidden in the clue.
'NYLON' is hidden in the letters of 'many longbows'.
'you'll find' is the link.
(Other definitions for nylon that I've seen before include "Plastic used for stockings" , "Stocking; polymer" , "Synthetic material used in textiles" , "Synthetic polymer" , "Man-made material" .)