Boundary area around the Central line (4)
I believe the answer is:
axis
'the central line' is the definition.
(I've seen this before)
'boundary area around' is the wordplay.
'boundary' becomes 'six' (type of score in cricket).
'area' becomes 'a' (maths abbreviation).
'around' is a reversal indicator.
'six'+'a'='sixa'
'sixa' in reverse letter order is 'AXIS'.
(Other definitions for axis that I've seen before include "Line of symmetry" , "On which a body spins" , "Group of countries in special alliance" , "Center around which something rotates" , "(Rotational?) alliance of powers" .)