In the middle of the red line (7)
I believe the answer is:
central
This clue is a double definition.
'in the middle' is the first definition.
(I've seen this before)
'the red line' is the second definition.
(red line on London Underground maps)
'of' acts as a link.
(Other definitions for central that I've seen before include "Focal" , "Most important, principal" , "Key" , "Middle; crucial" , "Every answer here has one such letter" .)