Grain store left overturned on outskirts of Maine's largest city (10)

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

SILO (grain store) + PORT (left, in nautical terminology), all reversed (overturned), after the outer letters (outskirts) of M [ain] E . A large city, considered to be a significant economic or political centre in its region.