In slang LGBT "clocked means to be identified as a transgender or recognized as a transgender, often against a person's will or when they are trying to blend in with others.
Clocked in slang LGBT implies that someone has noticed or has understood that a person is transgender, even if that person is not stating it explicitly or wants to keep it private.
"Clocked" specifically refers to the moment that someone realizes that another person is transgender, especially in situations in which the transgender person is trying to present as cisgender.
And in many contexts, "clocked" also carries a negative connotation, most particularly for people who are trying to maintain their sense of stealth or privacy about their status as being transgender.
It can also be upsetting or even dangerous if someone is "clocked" in a way which leads to harassment or unwanted attention.
The concept of "clocking" is closely related to the idea of "passing," which also refers to the ability of a transgender person to be perceived as cisgender.
And if someone is "clocked," it also means that their attempts have failed.