Seems to be used more in the gay community, more specifically in the gay black community.
T is short for "the truth," so clocking the T means
1) to tell the truth, the whole truth, and nothing but the truth, without embellishments, without edits, and without apology.
2) to state your own personal opinion about any person, place, thing, or situation, again without embellishments, edits, or apology.
In layman's terms, Clocking the T is synonymous with "keeping it real." It can also be used for gossip.
Quaadir Howard from youtube (username Timaya) giving his opinion on Lady Gaga (paraphrased)-
"I don't too much care for her. People are giving her praises but I'm not impressed. Her videos and perfornances hardly make sense with her songs. She's vulgar and over the top just for the sake of getting attention and fame. That is my opinion and you don't have to like it. If she gave me what i needed, I would have given her kudos, but she didn't and I'm going to clock the T where the T needs to be clocked, mmkay sweetheart?"
Adam - "Watch this dance move!" *starts dancing horribly* "So how did you like my dancing, Keith?"
Keith - "You really don't need to be dancing. You look like a retarded flamingo trying to find a mate."
Adam - "that's not very nice"
Keith - "Well if you ask for my opinion, ms thing, I'm going to clock the T on that ass. how you doin."