Greywether's fieldnotes pretty much describe the chamber cairn perfectly.
However next the cairn there is a wheelhouse which was probably made the Iron Age folks recycling the nearby stones. Two of the wheel spokes or spokes almost remain in place.
To complicated for me describe, the best thing to do is to put up the Canmore link below.
It's a Clyde Cairn in the heart of Hebridean Passage Grave territory. Probably not the only Clyde Cairn in the area admittedly but the one with the most positive identification.
Clettraval consists of an unusually long five-compartment chamber facing E and a straight facade of fallen stones running N/S (best preserved in the S). An Iron Age wheel house, not very well preserved, has been built in the W end of the cairn. It was excavated in 1934 (see Link).
The views from here are splendid - even on a cloudy day. The cairn sits on he 100m contour and, to the S, are the low-lying Lochs Bhaghasaraidh and Steaphain.
Access is very easy. Drive up the metalled military road which runs E from the crossroads at NF718722 until you reach the first radio mast where there is parking. The cairn can be seen in the field opposite.
This 'forgotten' standing stone is only a short distance to the north west of the wheelhouse/chamber cairn. Only a small fence to jump.
Standing at 2m wide and 1.5m it gives us clear views to the cairn at Corary and the possible chamber cairn at Cleitreabhal A Tuath. Also on a very clear day, St. Kilda.
A fine start to wandering about the Clettraval sites.