  Colour-magnitude diagrams (CMD) of the SMC star cluster NGC419, derived from
HST/ACS data, reveal a well-delineated secondary clump located below the
classical compact red clump typical of intermediate-age populations. We
demonstrate that this feature belongs to the cluster itself, rather than to the
underlying SMC field. Then, we use synthetic CMDs to show that it corresponds
very well to the secondary clump predicted to appear as a result of He-ignition
in stars just massive enough to avoid electron-degeneracy settling in their
H-exhausted cores. The main red clump instead is made of the slightly less
massive stars which passed through electron-degeneracy and ignited He at the
tip of the RGB. In other words, NGC419 is the rare snapshot of a cluster while
undergoing the fast transition from classical to degenerate H-exhausted cores.
At this particular moment of a cluster's life, the colour distance between the
main sequence turn-off and the red clump(s) depends sensitively on the amount
of convective core overshooting, Lambda_c. By coupling measurements of this
colour separation with fits to the red clump morphology, we are able to
estimate simultaneously the cluster mean age (1.35(-0.04,+0.11) Gyr) and
overshooting efficiency (Lambda_c=0.47(-0.04,+0.14)). Therefore, clusters like
NGC419 may constitute important marks in the age scale of intermediate-age
populations. After eye inspection of other CMDs derived from HST/ACS data, we
suggest that the same secondary clump may also be present in the LMC clusters
NGC1751, 1783, 1806, 1846, 1852, and 1917.
