  The ionising radiation of young and massive stars is a crucial form of
stellar feedback. Most ionising (Lyman-continuum; LyC, $\lambda < 912A$)
photons are absorbed close to the stars that produce them, forming compact HII
regions, but some escape into the wider galaxy. Quantifying the fraction of LyC
photons that escape is an open problem. In this work, we present a semi-novel
method to estimate the escape fraction by combining broadband photometry of
star clusters from the Legacy ExtraGalactic UV Survey (LEGUS) with HII regions
observed by the Star formation, Ionized gas, and Nebular Abundances Legacy
Survey (SIGNALS) in the nearby spiral galaxy NGC 628. We first assess the
completeness of the combined catalogue, and find that 49\% of HII regions lack
corresponding star clusters as a result of a difference in the sensitivities of
the LEGUS and SIGNALS surveys. For HII regions that do have matching clusters,
we infer the escape fraction from the difference between the ionising power
required to produce the observed HII luminosity and the predicted ionising
photon output of their host star clusters; the latter is computed using a
combination of LEGUS photometric observations and a stochastic stellar
population synthesis code SLUG (Stochastically Lighting Up Galaxies). Overall,
we find an escape fraction of $f_{esc} = 0.09^{+0.06}_{-0.06}$ across our
sample of 42 HII regions; in particular, we find HII regions with high
$f_{esc}$ are predominantly regions with low H$\alpha$-luminosity. We also
report possible correlation between $f_{esc}$ and the emission lines [O ii]/[N
ii] and [O ii]/H$\beta$.
