Synthetic material needed by many Londoners (5)
I believe the answer is:
nylon
'synthetic material' is the definition.
(I've seen this in another clue)
'needed by many londoners' is the wordplay.
'needed by' indicates a hidden word (the solution letters are needed to write the clue).
'NYLON' can be found hidden inside 'many londoners'.
(Other definitions for nylon that I've seen before include "may be damaged by running" , "Unaccompanied part song" , "Cover for leg" , "Artificial cloth" , "Synthetic fabric invented in 1938" .)