Basic line taken in London (7)
I believe the answer is:
central
'basic' is the definition.
(I've seen this in another clue)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for central that I've seen before include "Every answer here has one such letter" , "Near the middle" , "like this answer" , "Focal" , "Key" .)