Large city is withdrawing source of power (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A reversal (withdrawing) of WEN (large, overcrowded city) + IS In the 1820s, William Cobbett, in his ‘Rural Rides’, coined the phrase ‘The Great Wen’ to describe London, as he saw the rapidly growing city as a pathological swelling on the face of the nation – a “wen” is a sebaceous cyst – cf the then Prince Charles’ ‘monstrous carbuncle’ speech )