Stars make much use of unknown route (5,3)
I believe the answer is:
milky way
'stars' is the definition.
'much use of unknown route' is the wordplay.
'much use of' becomes 'milk' (I am not sure about this - if you are sure you should believe this answer much more).
'unknown' becomes 'y' (algebra).
'route' becomes 'way' (way is a kind of route).
'milk'+'y'+'way'='MILKY WAY'
'make' acts as a link.
Can you help me to learn more?
(Other definitions for milky way that I've seen before include "Lots of stars" , "Galaxy, broad faintly luminous band of stars" , "Star-studded band" , "Galaxy containing our solar system" , "Millions of stars in a band of light across the night sky" .)