  We report the serendipitous detection of a significant overdensity of
Herschel-SPIRE 250 micron sources in the vicinity of MRC1138-26. We use an
adaptive kernel density estimate to quantify the significance, including a
comparison with other fields. The overdensity has a size of ~3.5-4' and stands
out at ~5sigma with respect to the background estimate. No features with
similar significance were found in four extragalactic control fields:
GOODS-North, Lockman, COSMOS and UDS. The chance of having a similar
overdensity in a field with the same number but randomly distributed sources is
less than 2%. The clump is also visible as a low surface brightness feature in
the Planck 857 GHz map. We detect 76 sources at 250 micron (with a
signal-to-noise ratio greater than 3), in a region of 4' radius; 43 of those
are above a flux density limit of 20 mJy. This is a factor of 3.6 in excess
over the average in the four control fields, considering only the sources above
20 mJy. We also find an excess in the number counts of sources with 250 micron
flux densities between 30 and 40 mJy, compared to deep extragalactic
blank-field number counts. Assuming a fixed dust temperature (30 K) and
emissivity (beta=1.5) a crude, blackbody-derived redshift distribution, zBB, of
the detected sources is significantly different from the distributions in the
control fields and exhibits a significant peak at zBB ~ 1.5, although the
actual peak redshift is highly degenerate with the temperature. We tentatively
suggest, based on zBB and the similar S250/S350 colours of the sources within
the peak, that a significant fraction of the sources in the clump may be at a
similar redshift. Since the overdensity lies ~7' south of the z=2.16 Spiderweb
protocluster MRC1138-26, an intriguing possibility (that is presently
unverifiable given the data in hand) is that it lies within the same
large-scale structure.(abridged)
