  Aims: We study a peculiar object with a projected position close to the
nucleus of M51. It is unusually large for a star cluster in M51 and we
therefore investigate the three most likely options to explain this object: (a)
a background galaxy, (b) a cluster in the disk of M51 and (c) a cluster in M51,
but in front of the disk. Methods: We use HST/ACS and HST/NICMOS broad-band
photometry to study the properties of this object. Assuming the object is a
star cluster, we fit the metallicity, age, mass and extinction using simple
stellar population models. Assuming the object is a background galaxy, we
estimate the extinction from the colour of the background around the object. We
study the structural parameters of the object by fitting the spatial profile
with analytical models. Results: We find de-reddened colours of the object
which are bluer than expected for a typical elliptical galaxy, and the central
surface brightness is brighter than the typical surface brightness of a disc
galaxy. It is therefore not likely that the object is a background galaxy.
Assuming the object is a star cluster in the disc of M51, we estimate an age
and mass of 0.7 Gyr and 2.2 x 10^5 \msun, respectively (with the extinction
fixed to E(B-V) = 0.2). Considering the large size of the object, we argue that
in this scenario we observe the cluster just prior to final dissolution. If we
fit for the extinction as a free parameter, a younger age is allowed and the
object is not close to final dissolution. Alternatively, the object could be a
star cluster in M51, but in front of the disc, with an age of 1.4 Gyr and mass
M = 1.7 x 10^5 \msun. Its effective radius is between ~12-25 pc. This makes the
object a "fuzzy star cluster", raising the issue of how an object of this age
would end up outside the disc.
