Material held by many longshoremen (5)
I believe the answer is:
nylon
'material' is the definition.
(nylon is a kind of material)
'held by many longshoremen' is the wordplay.
'held by' indicates a hidden word.
'NYLON' is hidden in the letters of 'many longshoremen'.
(Other definitions for nylon that I've seen before include "Artificial yarn/fabric" , "Man-made fibre" , "Plastic used for stockings" , "Yarn for stockings" , "Synthetic polymer" .)