A political line? (4)
I believe the answer is:
axis
'line?' is the definition.
(axis is a kind of line)
'a political' is the wordplay.
'political' becomes 'xis' (I can't explain this - if you can you should believe this answer much more).
'a'+'xis'='AXIS'
Can you help me to learn more?
(Other definitions for axis that I've seen before include "x or y line on a graph" , "On which a body spins" , "Imaginary centre of rotation" , "where wheel turns?" , "coalition" .)