Material used by many Londoners (5)
I believe the answer is:
nylon
'material' is the definition.
(nylon is a kind of material)
'used by many londoners' is the wordplay.
'used by' says the answer is hidden in the clue.
'NYLON' can be found hidden inside 'many londoners'.
(Other definitions for nylon that I've seen before include "Hosiery material" , "Fibre of polymeric amides" , "Plastic used for stockings" , "Strong, resilient, synthetic fibre" , "Artificial fibre" .)