Material held by many Londoners (5)
I believe the answer is:
nylon
'material' is the definition.
(nylon is a kind of material)
'held by many londoners' is the wordplay.
'held by' indicates the answer is hidden within the clue.
'NYLON' is hidden within 'many londoners'.
(Other definitions for nylon that I've seen before include "Strong, resilient, synthetic fibre" , "Stocking; polymer" , "Hose" , "Tights material" , "Man-made fibre" .)