We analyse the first publicly released deep field of the UK Infrared Deep Sky Survey (UKIDSS) Deep eXtragalactic Survey to identify candidate galaxy overdensities at z similar to 1 across similar to 1 deg(2) in the ELAIS-N1 field. Using I - K, J - K and K - 3.6 mu m colours, we identify and spectroscopically follow up five candidate structures with Gemini/Gemini Multi-Object Spectrograph and confirm that they are all true overdensities with between five and 19 members each. Surprisingly, all five structures lie in a narrow redshift range at z = 0.89 +/- 0.01, although they are spread across 30 Mpc on the sky. We also find a more distant overdensity at z = 1.09 in one of the spectroscopic survey regions. These five overdense regions lying in a narrow redshift range indicate the presence of a supercluster in this field and by comparing with mock cluster catalogues from N-body simulations we discuss the likely properties of this structure. Overall, we show that the properties of this supercluster are similar to the well-studied Shapley and Hercules superclusters at lower redshift.