I want to add a reverse reverb to only a part of a track. Is this possible?

Yes. There's actually an FX bin for clips.

You can put an effect right into the clip FX bin and the effect only kicks in for that particular clip.

I have no idea what a clip is, suppose its reading time. Thanks.

A clip is where your recorded audio resides within the Track Pane in Sonar
 
A given Track in Sonar can consist of one or many clips
 
In order to apply clip Fx to a small section you need to split the desired clip at the start & end points

Thanks, i'll try to split it.

Another alternative is to set up a buss with the reverse reverb on and then use automation to control when it is sent there.
 
Clip FX are better for the odd clip, if it's something you want more than one instance of through the course of a track an automated buss is probably the better way to go, especially if you want different levels of it at different points.

If you want to add DSP-style reverse reverb by actually reversing the waveform, and not just use a reverse reverb effect, you can simply drag across the part of the clip you want to process (I don't think this works with a Groove Clip, though, you need to bounce it to itself so it becomes a standard clip) and choose Process > Apply Effect > Reverse.
 
Note that reversing will probably mess up the timing. I create a separate track for reversed clips so I can nudge them as desired to line up with the tempo.

Question re this please....if I add the FX to a clip, does this then override the main FX bin or does it run in parallel? Further, after that clip is finished does the FX in the main bin re-engage for other clips on the track?
 
Thanks

The effects in the FX bins are applied in series not parallel. Effects in one bin do not "override" another. The signal originates in the clips and passes through the bins in this order:

• Region FX
• Clip FX bin
• ProChannel (by default)
• Track FX bin
• ProChannel (if set to Post FX bin)

 
The ProChannel only exists in one place on each track. Each track has a pre/post Track FX bin switch. Refer to the signal flow diagram http://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=Mixing.07.html
 

It runs in parallel. The Clip FX bin has no effect on the track FX bin.