It seems that in Clearspace 2.5.5 , discussion thread view is changed.
For eg. In earier version An arrow use to point to its child discussion thread.
Please go by the attached images.
Now if i have to implement that view ( earlier one ), What are all steps that i need to follow ?
It looks like you've got thread pagination enabled. You'll want to go here:
Your Stuff -> Preferences -> Thread Page View: Threaded