Sloth in tree mostly climbing with another likewise (7)
I believe the answer is:
inertia
'sloth' is the definition.
(inertia can mean laziness or sloth)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for inertia that I've seen before include "Tendency to inaction" , "Stillness" , "Resistance to movement" , "Motion (if any) when no external force is applied" , "Disinclination to change" .)