A cross is in line (4)

I believe the answer is:

axis

'line' is the definition.
(axis is a kind of line)

'a cross is' is the wordplay.
'cross' becomes 'x'.
'a'+'x'+'is'='AXIS'

'in' acts as a link.

(Other definitions for axis that I've seen before include "Line from N. to S. Pole" , "Turning point" , "Line around which something rotates" , "Pivot, political alliance" , "Line of symmetry" .)

I've seen this clue in the Evening Standard.

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