  ASASSN-21qj is a distant Sun-like star that recently began an episode of deep
dimming events after no prior recorded variability. Here we examine archival
and newly obtained optical and near-infrared data of this star. The deep
aperiodic dimming and absence of previous infrared excess are reminiscent of
KIC 8462852 (``Boyajian's Star''). The observed occultations are consistent
with a circumstellar cloud of sub-micron-sized dust grains composed of
amorphous pyroxene, with a minimum mass of
$1.50~\pm~0.04\times10^{-9}~M_{\oplus}$ derived from the deepest occultations,
and a minimum grain size of $0.29^{+0.01}_{-0.18}~\mu$m assuming a power law
size distribution. We further identify the first evidence of near-infrared
excess in this system from NEOWISE 3.4 and 4.6~$\mu$m observations. The excess
emission implies a total circumstellar dust mass of around $10^{-6}
M_{\oplus}$, comparable to the extreme, variable discs associated with
terrestrial planet formation around young stars. The quasiperiodic recurrence
of deep dips and the inferred dust temperature (ranging from 1800 to 700~K
across the span of observations) independently point to an orbital distance of
$\simeq$0.2~au for the dust, supporting the occulting material and excess
emission being causally linked. The origin of this extended, opaque cloud is
surmised to be the breakup of one or more exocometary bodies.
