  The Hubble Frontier Fields (HFF) are a selection of well-studied galaxy
clusters used to probe dense environments and distant gravitationally lensed
galaxies. We explore the 21cm neutral hydrogen (HI) content of galaxies in
three of the HFF clusters, Abell 2744 ($z = 0.308$), Abell S1063 ($z = 0.346$)
and Abell 370 ($z = 0.375$), to investigate the evolution of gas in galaxies
within intermediate redshift clusters. Using Early Science MeerKAT
observations, we perform spectral-line stacking with HI cubes and make a
3$\sigma$ stacked detection for blue galaxies in Abell S1063 ($M_\mathrm{HI} =
1.22^{+0.38}_{-0.36}\times 10^{10} \mathrm{M}_\odot$). We determine the
3$\sigma$ HI mass detection limits of Abell 2744 and Abell 370 to be at the
knee of the HI Mass Function. A final, more ambitious objective of this work is
to search for gravitationally lensed HI emission behind these clusters, enabled
by MeerKAT's wide instantaneous bandwidth. We find no evidence of highly
magnified HI emission at $0.33<z<0.58$. The low thermal noise levels achieved
in these pilot observations, despite short integration times, highlights the
enormous potential of future MeerKAT HI observations of dense environments and
the intermediate-redshift Universe.
