Lunar features in large part of planet I start to admit (5)

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

Most of (large part of) MAR [s] (a planet) + I (from clue text) + starting letter of A [dmit]. Dark areas on the Moon’s surface, named from the Latin word for “seas”, because that’s what people thought they were before we had the technology to explore them properly.