London area cannot shift first geometric lines (7)

I believe the answer is:

secants

'geometric lines' is the definition.
(I've seen this in another clue)

'london area cannot shift first' is the wordplay.
'london area' becomes 'SE' (postcode for south east London).
'cannot' becomes 'can't' (contraction).
'shift first' becomes 's' (1st letter of 'shift').
'se'+'cant'+'s'='SECANTS'

I've seen this clue in The Daily Express.

Want a hint initially instead of a full solution? Install my app