There's little change in one (3)
I believe the answer is:
rut
'change in one' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe there's a link between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for rut that I've seen before include "Groove worn in surface" , "Groove or furrow - don't get into one" , "It can be frustrating" , "animal passion" , "hazard in the road" .)