Texas does not actually have a clause to secede, and the Supreme Court settled the case in Texas v. White (1869) that Texas, or other states, do not have the right to secede (and that furthermore, despite the withdrawals of their Congress members, the states never seceded in the first place).

The actual clause that Texas has in its annexation resolution is this clause granting Texas the ability to create up to four more states out of its land:

Third. New States, of convenient size, not exceeding four in number, in addition to said State of Texas, and having sufficient population, may hereafter, by the consent of said State, be formed out of the territory thereof, which shall be entitled to admission under the provisions of the federal constitution.

This is a mainly historical movement called Texas divisionism and there aren’t any serious advocates of splitting Texas into any other states today as far as I know.