。
It's already 11 o'clock, you know.
Literal
11 o'clock [target-に] becoming [emphasis-よ].
Note なっている — the resultative ~ている of なる ('become') describes the state of *having become* 11 o'clock, not the moment of becoming. So the sentence frames the time as 'we are now in the state of it being 11.' This is a key use of ~ている: with change-of-state verbs, it marks the resulting state. The よ delivers it as fresh information.