Central Line for Cockney has eleven stops (4)
I believe the answer is:
axis
'central line' is the definition.
(I've seen this before)
'cockney has eleven stops' is the wordplay.
'cockney has' becomes ''as' ('has' said with the 'H' dropped - a cockney accent.).
'eleven' becomes 'xi' (Roman numerals).
'stops' means one lot of letters goes inside another (stopping can mean blocking a gap).
'as' placed around 'xi' is 'AXIS'.
'for' is the link.
(Other definitions for axis that I've seen before include "Line of symmetry" , "wartime alliance" , "WW2 Nazi alliance" , "line to be turned round" , "Imaginary line about which a body rotates and a Dublin theatre" .)