  We report the detection of a significant excess in the surface density of
far-infrared sources from the Herschel-Astrophysical Terahertz Large Area
Survey (H-ATLAS) within ~1 Mpc of the centres of 66 optically-selected clusters
of galaxies in the SDSS with <z>~0.25. From the analysis of the multiwavelength
properties of their counterparts we conclude that the far-infrared emission is
associated with dust-obscured star formation and/or active galactic nuclei
within galaxies in the clusters themselves. The excess reaches a maximum at a
radius of ~0.8 Mpc, where we find 1.0\pm0.3 S_250um>34 mJy sources on average
per cluster above what would be expected for random field locations. If the
far-infrared emission is dominated by star formation (as opposed to AGN) then
this corresponds to an average star formation rate of ~7 M_sun/yr per cluster
in sources with L_IR>5d10 L_sun. Although lensed sources make a negligible
contribution to the excess signal, a fraction of the sources around the
clusters could be gravitationally lensed, and we have identified a sample of
potential cases of cluster-lensed Herschel sources that could be targeted in
follow-up studies.
