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 the answer is hidden within the clue (the solution letters are needed to write the clue).
'NYLON' is hidden within '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" .)