The Clandestine Betrothal — Alice Chetwynd Ley

Girlish, defiant, she had to be a wife...but longed to be a woman.

Love in Name Only

When Susan announced to her aunt that she was engaged to Beau Eversley, the most sought-after bachelor in London, she should have been the happiest girl in England. The trouble was the Beau knew nothing of the betrothal. Susan had made up the whole affair. And to make matters worse, she would soon be found out.

There was nothing to do but confess her outrageous falsehood to the handsome Beau. But once she did, he agreed to play along with the charade. It seemed the perfect solution, until Susan suddenly found herself trapped in a lie, and engaged in name only to the man she truly loved.
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Corgi (UK) Edition:

Released at last from her seminary for young ladies, Susan Fyfield hoped to find romance and excitement now that the schoolroom was behind her. At the centre of her dreams was the Hon. Hugh Eversley, the dashing, elusive brother of her friend, Georgiana. But Beau Eversley was the talk of the town and eagerly pursued by all the matchmaking mamas of the London season.

At Georgiana's coming-out party, Susan danced with the Beau, had her first glass of champagne, and then behaved rather indiscreetly. The next day she sought out the Beau and confessed to having told her guardian aunt of an imaginary betrothal between them. But far from reacting as she had expected, Hugh Eversleu suggested that the engagement should become reality...
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Robert Hale (Hardcover) First Edition:

When Susan Fyfield, newly arrived at her aunt's house in London from a Young Ladies' Seminary in Twickenham says that she is secretly betrothed to the most eligible and elusive bachelor in Town, it is hardly surprising that her relatives do not believe her. Beau Eversley's flirtations are the despair of every match-making Mama, while all the Town has been gossiping of his more dubious amorous adventures with such ladies as the vivacious actress Maria McCann.

Subsequent events cause Mrs Fyfield to treat the affair more seriously. As a result, Susan finds herself the centre of a mystery which threatens her happiness. Aided by her school friend, the Beau's volatile younger sister Georgiana, Susan sets out to solve the problem. The renowned Horace Walpole, Society's foremost gossip, eventually supplies the key to the mystery, and the story ends as it began, in his strange Gothic villa at Twickenham.