  We present the first measurement of HI mass of star-forming galaxies in
different large scale structure environments from a blind survey at $z\sim
0.37$. In particular, we carry out a spectral line stacking analysis
considering $2875$ spectra of colour-selected star-forming galaxies undetected
in HI at $0.23 < z < 0.49$ in the COSMOS field, extracted from the MIGHTEE-HI
Early Science datacubes, acquired with the MeerKAT radio telescope. We stack
galaxies belonging to different subsamples depending on three different
definitions of large scale structure environment: local galaxy overdensity,
position inside the host dark matter halo (central, satellite, or isolated),
and cosmic web type (field, filament, or knot). We first stack the full
star-forming galaxy sample and find a robust HI detection yielding an average
galaxy HI mass of $M_{\rm HI}=(8.12\pm 0.75)\times 10^9\, {\rm M}_\odot$ at
$\sim 11.8\sigma$. Next, we investigate the different subsamples finding a
negligible difference in $M_{\rm HI}$ as a function of the galaxy overdensity.
We report an HI excess compared to the full sample in satellite galaxies
($M_{\rm HI}=(11.31\pm1.22)\times 10^9$, at $\sim 10.2 \sigma$) and in
filaments ($M_{\rm HI}=(11.62\pm 0.90)\times 10^9$. Conversely, we report
non-detections for the central and knot galaxies subsamples, which appear to be
HI-deficient. We find the same qualitative results also when stacking in units
of HI fraction ($f_{\rm HI}$). We conclude that the HI amount in star-forming
galaxies at the studied redshifts correlates with the large scale structure
environment.
