Large city is withdrawing source of power (5)
I believe the answer is:
sinew
'source of power' is the definition.
(sinew can mean a source of power or strength)
'large city is withdrawing' is the wordplay.
'large city' becomes 'wen' (historical term for a large city).
'withdrawing' shows that the letters should be reversed in order.
'wen'+'is'='wenis'
'wenis' back-to-front is 'SINEW'.
(Other definitions for sinew that I've seen before include "Tendon - ligament" , "power source" , "Tough material" , "Muscular strength" , "Force" .)