Synthetic material used in many longboats (5)
I believe the answer is:
nylon
'synthetic material' is the definition.
(I've seen this before)
'in many longboats' is the wordplay.
'in' says the answer is hidden in the clue.
'NYLON' is hidden within 'many longboats'.
'used' acts as a link.
(Other definitions for nylon that I've seen before include "Artificial fibre" , "Unaccompanied part song" , "Polyamide" , "Tough synthetic material" , "Cover for leg" .)