brakhorloĝo

Two roots, no glue. Esperanto compounds by writing the modifier first and the head second, and joining them directly: brako arm plus horloĝo clock gives brakhorloĝo, a clock for the arm. The head noun is the one on the right, which is why this is a kind of clock and not a kind of arm.

Notice what got dropped. Brako loses its -o at the join, because the ending belongs to the finished word, not to each piece. Sometimes the vowel stays for the sake of the ear - vaporŝipo keeps its o where vaprŝipo would be unsayable - and the rule is nothing more principled than that: keep it if the cluster is hard, drop it if it is not. Brakh- is fine to say, so the o goes.

The pattern is worth practising because it is how Esperanto names most modern objects. Poŝhorloĝo is a pocket watch, sonorilhorloĝo an alarm clock, sunhorloĝo a sundial. In each case the second root tells you what the thing is and the first tells you which sort. And if the compound ever feels heavy, the prepositional escape route is always open: horloĝo por la brako says the same thing, at more length.