Being in one can mean lengthy depression (3)
I believe the answer is:
rut
'depression' is the definition.
(rut is a kind of depression)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rut that I've seen before include "Well-worn track" , "humdrum activity" , "Wheel-furrow" , "Kind of groove or fixed habit" , "Groovy thing" .)