seruro

No suffix, whatever it looks like. Seruro ends in -uro, and a learner who has been enjoying Esperanto's transparency will try to cut it: ser- plus -ur-. That fails twice over. There is no -ur- suffix in Esperanto - the official list has -ad-, -aĵ-, -an-, -ar-, -ec-, -eg-, -ej- and the rest, and -ur- is not among them - and ser- is not a root either. Whole word, straight from French serrure, and the -o is the only Esperanto part of it.

Three words for one operation, and they are not all the same root. The seruro is the mechanism in the door. The ŝlosilo is the key you put in it. And ŝlosi is the verb, to lock. Note that the key and the verb share a root - ŝlos-, from German schliessen - while the lock does not. So ŝlosilo decodes as the tool for locking with the -il- suffix, and seruro decodes as nothing. It is a small untidiness in a very tidy language, and it is exactly the kind of thing worth noticing rather than glossing over.

Everyday phrases. Ŝlosi per ŝlosilo is redundant and people say it anyway. La pordo estas sub seruro means it is locked up. A serurist(in)o is a locksmith, using the trade suffix -ist-, and senserura describes a door with no lock at all.

Where you meet it. Doors, gates, suitcases, bicycles, a kesto in the attic and, in older texts, the strongbox in the corner of a merchant's office. If the thing keeps something sekura, the word you want is probably this one.