Tough resistance shown by pair coming in down under? (7)

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

[ R(symbol for “electrical resistance” in physics) plus ( by ) DUO(a pair/a duet in music or entertainment) ] contained in ( coming in ) AUS(abbrev. for “Australia”, a country refered to as “down under” by those in the UK).