  We present a new fast radio burst (FRB) at 920 MHz discovered during
commensal observations conducted with the Australian Square Kilometre Array
Pathfinder (ASKAP) as part of the Commensal Real-time ASKAP Fast Transients
(CRAFT) survey. FRB 191001 was detected at a dispersion measure (DM) of
506.92(4) pc cm$^{-3}$ and its measured fluence of 143(15) Jy ms is the highest
of the bursts localized to host galaxies by ASKAP to date. The subarcsecond
localization of the FRB provided by ASKAP reveals that the burst originated in
the outskirts of a highly star-forming spiral in a galaxy pair at redshift
$z=0.2340(1)$. Radio observations show no evidence for a compact persistent
radio source associated with the FRB 191001 above a flux density of $15 \mu$Jy.
However, we detect diffuse synchrotron radio emission from the disk of the host
galaxy that we ascribe to ongoing star formation. FRB 191001 was also detected
as an image-plane transient in a single 10 s snapshot with a flux density of
19.3 mJy in the low-time-resolution visibilities obtained simultaneously with
CRAFT data. The commensal observation facilitated a search for repeating and
slowly varying radio emissions 8 hr before and 1 hr after the burst. We found
no variable radio emission on timescales ranging from 1 ms to 1.4 hr. We report
our upper limits and briefly review FRB progenitor theories in the literature
that predict radio afterglows. Our data are still only weakly constraining of
any afterglows at the redshift of the FRB. Future commensal observations of
more nearby and bright FRBs will potentially provide stronger constraints.
