Which rocket first used clustering of liquid fueled rocket engines on their first stage?
I find a few different candidates depending on your definitions.
1946: The Bell X-1 rocket plane used the XLR-11, a 4-chamber, pressure-fed rocket engine using liquid propellants. The four chambers were integrated into a single module, so it should probably be considered a single engine rather than a cluster, but the four chambers could be individually turned on and off. In theory the X-1 could take off from a runway, but in practice it was always dropped from a B-29 mothership, so it's also debatable whether this counts as a first stage.
1957: The R-7 ICBM used the RD-107/RD-108 engine on its four boosters and core stage. This is an engine with four combustion chambers, but driven by a common turbopump, which by convention is considered a single engine, so it's not technically a cluster. If you consider the four individual boosters as a single stage, that would be a four-engine cluster.
1961: The Saturn I used a cluster of 8 H-1 engines on its first stage (and a cluster of 6 RL10 engines on the second stage). This is the earliest unambiguous example of a liquid-propellant clustered first stage I could find.
There are probably other experimental examples of clustered engines. The Wikipedia article on Robert Goddard mentions a "3000-lb thrust engine using a cluster of four 750-lb thrust motors" built at Caltech; this would have been in the late '40s or early '50s, but I can't find any more information on it, so I don't know if it was ever associated with a particular rocket stage.