When one who is inexperienced and/or not completely fit for the task at hand is given a chance to try it without assistance from a more experienced person, they are being given their deserved third string solo.
Phil: I'll be away Friday, so the intern will have to finish the project proposal.
Clyde: But he's only an intern! Shouldn't George help him with it?
Phil: Oh, relax, he deserves a third string solo.