There are papers out there that qualify chloroquine and derivates as effective in case of viral infections with viruses related to covid-19 [0] and try to explain what the mechanism could be :

« The exact mechanism of antiviral intervention by chloroquine is not yet elucidated but is possibly a multitarget mechanism, depending on the time point at which the drug is added »

[0] https://aac.asm.org/content/53/8/3416