der Wecker
alarm clock
A clock or device that makes a sound at a preset time in order to wake someone up.
Mein Wecker hat heute Morgen um sechs Uhr geklingelt.
My alarm clock rang at six o'clock this morning.

alarm clock
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alarm clock
A clock or device that makes a sound at a preset time in order to wake someone up.
Mein Wecker hat heute Morgen um sechs Uhr geklingelt.
My alarm clock rang at six o'clock this morning.

The noun 'der Wecker' (alarm clock) is derived directly from the verb 'wecken' (to wake someone up). So a Wecker is literally an object that wakes someone up.
Kannst du mich morgen früh bitte wecken?
Could you please wake me up early tomorrow?
Three typical actions are associated with 'Wecker' (alarm clock): you can 'stellen' (set) the alarm clock, it 'klingelt' (rings) or 'geht los' (goes off) in the morning, and you can 'ausmachen' (turn it off).
Ich habe vergessen, den Wecker zu stellen.
I forgot to set the alarm clock.
When setting the time at which the alarm clock should ring, you use the preposition 'auf' followed by the accusative. You set the alarm clock 'auf' (for) a specific time.
Ich stelle meinen Wecker immer auf halb sieben.
I always set my alarm clock for six thirty.
clock / watch
'Uhr' and 'Wecker' are both devices that have to do with time. A 'Uhr' shows the time, for example on a wall or on your wrist. A 'Wecker' is a special kind of clock or device: it makes a sound at a set time so that someone wakes up. They can be confused because an alarm clock can also show the time. If only the time is important, you say 'Uhr'. If waking someone up is important, you say 'Wecker'.

Heute stelle ich den Wecker auf sechs Uhr.
Today I set the alarm clock for six o'clock.
Im Flur hängt eine große Uhr, damit alle die Zeit sehen können.
There is a large clock hanging in the hallway so that everyone can see the time.