  Context. Intrinsic H$\alpha$ emission can be advantageously used to detect
substellar companions because it improves contrasts in direct imaging.
Characterising this emission from accreting exoplanets allows for the testing
of planet formation theories.
  Aims. We characterise the young circumbinary planetary mass companion 2MASS
J01033563-5515561 (AB)b (Delorme 1 (AB)b) through medium-resolution
spectroscopy.
  Methods. We used the new narrow-field mode (NFM) for the MUSE integral-field
spectrograph, located on the ESO Very Large Telescope, during science
verification time to obtain optical spectra of Delorme 1 (AB)b.
  Results. We report the discovery of very strong H$\alpha$ and H$\beta$
emission, accompanied by He I emission. This is consistent with an active
accretion scenario. We provide accretion rate estimates obtained from several
independent methods and find a likely mass of $12-15$ M$_{\rm Jup}$ for Delorme
1 (AB)b. This is also consistent with previous estimates.
  Conclusions. Signs of active accretion in the Delorme 1 system might indicate
a younger age than the $\sim 30-40$ Myr expected from a likely membership in
Tucana-Horologium (THA). Previous works have also shown the central binary to
be overluminous, which gives further indication of a younger age. However,
recent discoveries of active discs in relatively old ($\sim 40$ Myr), very
low-mass systems suggests that ongoing accretion in Delorme 1 (AB)b might not
require in and of itself that the system is younger than the age implied by its
THA membership.
