  We report the discovery of a transient seen in a strongly lensed arc at
redshift $z_{\rm s}=1.2567$ in \emph{Hubble Space Telescope} imaging of the
Abell 370 galaxy cluster. The transient is detected at $29.51\pm0.14$ AB mag in
a WFC3/UVIS F200LP difference image made using observations from two different
epochs, obtained in the framework of the \emph{Flashlights} program, and is
also visible in the F350LP band ($m_{\rm F350LP} \approx 30.53\pm0.76$ AB mag).
The transient is observed on the negative-parity side of the critical curve at
a distance of $\sim 0.6"$ from it, greater than previous examples of lensed
stars. The large distance from the critical curve yields a significantly
smaller macromagnification, but our simulations show that bright, O/B-type
supergiants can reach sufficiently high magnifications to be seen at the
observed position and magnitude. In addition, the observed transient image is a
trailing image with an observer-frame time delay of $\sim+0.8$ days from its
expected counterpart, so that any transient lasting for longer than that should
have also been seen on the minima side and is thus excluded. This, together
with the blue colour we measure for the transient ($m_{\rm F200LP} - m_{\rm
F350LP} \approx [-0.3,-1.6]$ AB), rules out most other transient candidates
such as (kilo)novae, for example, and makes a lensed star the prime candidate.
Assuming the transient is indeed a lensed star as suggested, many more such
events should be detected in the near future in cluster surveys with the
\emph{Hubble Space Telescope} and \emph{James Webb Space Telescope}.
