Material used in many longerons (5)
I believe the answer is:
nylon
'material' is the definition.
(nylon is a kind of material)
'in many longerons' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NYLON' is hidden within 'many longerons'.
'used' is the link.
(Other definitions for nylon that I've seen before include "Man-made fibre" , "Type of material" , "Tough synthetic material" , "Fibrous material" , "Tights material" .)