Dust jacket is missing but otherwise perfect (besides age). Teen fiction. Hetty Feather #4.
 
"Diamond wasn’t always a star. Born to penniless parents who longed for a strong, healthy son, she was a dainty, delicate daughter - and a bitter disappointment.
Discovering she has an extraordinary gift for acrobatics, Diamond uses her talent to earn a few pennies, but brings shame on her family. Then a mysterious, cruel-eyed stranger spots her performing, and makes a deal with her father. Diamond is sold for five guineas, and is taken to become an acrobat at Tanglefield’s Travelling Circus.
The crowds adore Diamond, but life behind the velvet curtains is far from glamorous. Her wicked master forces Diamond to attempt ever more daring and dangerous tricks, until she is terrified to step into the ring. But there are true friends to be found at the circus, too: the gentle Mister Marvel; the kindly Madame Adeline; and the glorious Emerald Star, Tanglefield’s brand-new ringmaster, and Diamond’s heroine.
When life at the circus becomes too dangerous to bear any longer, what will the future hold for Diamond? And will her beloved Emerald be a part of it?"