What map line may show, a route winding across Quebec (7)
I believe the answer is:
equator
'what map line' is the definition.
(I've seen this before)
'a route winding across quebec' is the wordplay.
'winding' indicates anagramming the letters.
'across' indicates putting letters inside.
'quebec' becomes 'Q' (phonetic alphabet: alpha, bravo, charlie etc.).
'a'+'route'='aroute'
'aroute' is an anagram of 'euator'.
'euator' enclosing 'q' is 'EQUATOR'.
'may show' acts as a link.
(Other definitions for equator that I've seen before include "Line separating northern and southern hemispheres" , "It's equidistant from North and South Poles" , "Circle round the globe" , "Imaginary line dividing the earth's hemispheres" , "Quorate (anag.)" .)