Yes, the sensor can be seen with truesight
I will briefly state what has already been pointed out in Dinomaster's answer:
The clairvoyance spell specifies that it creates "an invisible sensor", and truesight it described as allowing a creature to "see invisible creatures and objects", so this implies that such a sensor can be seen via truesight, although since a sensor isn't explicitly described as an object, it's not conclusive evidence...
In addition to that, a similar situation exists for the spell scrying (PHB, pg. 273), which is made more explicit:
... the spell creates an invisible sensor within 10 feet of the target. [...] A creature that can see invisible objects sees the sensor as a luminous object about the size of your fist.
So not only can creatures with truesight (or any creature that can see invisible objects for any other reason) see the sensor, but according to scrying, it looks like "a luminous objects about the size of your fist".
Given that the language regarding the "invisible sensor" is the same in both spells, it seems as though invisible sensors can be seen by creatures with truesight, and furthermore it's likely that the appearance of the sensor described by scrying would be the same for clairvoyance as well (or at the very least it gives the DM a precedent to follow).