Context : Classical novae ( CNe ) represent a major class of supersoft X-ray sources ( SSSs ) in the central region of our neighbouring galaxy M 31 .
Significantly different SSS properties of CNe in the M 31 bulge and disk were indicated by recent X-ray population studies , which however considered only a small number of disk novae .

Aims : We initiated a target of opportunity ( ToO ) program with XMM-Newton to observe the SSS phases of CNe in the disk of M 31 and improve the database for further population studies .

Methods : We analysed two XMM-Newton ToO observations triggered in Aug 2011 and Jan 2012 , respectively , and extracted X-ray spectra and light curves .

Results : We report the discovery of an X-ray counterpart to the M 31 disk nova M31N 2008-05d .
The X-ray spectrum of the object allows us to classify it as a SSS parametrised by a blackbody temperature of 32 \pm 6 eV .
More than three years after the nova outburst , the X-ray light curve of the SSS exhibits irregular , broad dip features .
These dips affect primarily the very soft part of the X-ray spectrum , which might indicate absorption effects .

Conclusions : Dipping SSS light curves are rarely observed in M 31 novae .
As well as providing an unparalleled statistical sample , the M 31 population of novae with SSS counterparts produces frequent discoveries of unusual objects , thereby underlining the importance of regular monitoring .