City wall with banana on? (9)

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

DAM (wall) after ROTTER (banana) Maybe – Are bananas particularly known for rotting? (at least they signal it by going black first) All fruit rots. Some quicker than bananas. Edit: I’ve been put right: Bananas rot faster than other fruit and give off ethylene (C₂H₄) that helps ripen and rot both themselves and other nearby fruit