Secret, covert, stealthy, furtive, clandestine, surreptitious, underhand, underhanded are comparable when they mean done, carried on, operated, or accomplished so as not to attract attention or observation.
Secret, the most general of these terms and the widest in its range of application, implies a hiding or concealing or a being hidden or concealed.
Covert applies to something that is done as it were under cover, and is not open or avowed.
Stealthy usually suggests an intent to elude, to spy upon, or to gain one’s ends without attracting attention; it is frequently a term either of derogation or of censure, connoting deliberateness and quietness in decoying, entrapping, or deceiving.
Furtive agrees with stealthy in suggesting an intent to escape observation but it carries clearer suggestions of cautiousness, watchfulness, or slyness, and is used to describe not only movements and acts but also faces, features, or expressions which reveal these or similar characteristics.
Clandestine implies concealment (as in working out a plan) and usually an evil or illicit end; it commonly suggests stealthy or furtive methods or a fear that others may know what is occurring.
Surreptitious applies not only to stealthy and furtive actions but also to emotions or desires and to concrete things which are concealed for fear of their discovery usually because they involve violation of a right, a law, a custom, or a standard (as of conduct or propriety).
Underhand and underhanded consistently carry an implication of fraud, deceit, or unfairness, in addition to that of secrecy in dealings or surreptitiousness in methods.