Sentence:  Monimia, who while she yielded to his earnest entreaties had always felt, from the natural rectitude of her understanding, the impropriety of their <WSD>clandestine</WSD> correspondence, would, he feared, be more than ever sensible of her indiscretion, when she found that a servant was entrusted with it... 
Meaning of the word marked with <WSD> tags:  Done or kept in secret, sometimes to conceal an illicit or improper purpose.