Gold rush essentially limiting blow for empire builder (8)

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

AU (Au = chemical symbol for gold, from Latin aurum ) + middle letters (essentially) from [r] US [h], containing (limiting) GUST (a gust of wind = blow, as a noun). The first Roman emperor, also known as Octavian; the name was also used as a title for later emperors.