The plural of clustlws is clustlysau, the free Welsh dictionary on iOS and Android, 'ap geiriaduron' is great for looking up things like this.
But is there a -d- in it or no? A bit of judicious Googling turns up a lot more for clustlws... I get that it's a contraction of clust + tlws, but which form is more commonly used?
All very well for those with mobile phones and i pads etc. I just have my windows laptop. Is there a dictionary I can use?