Union lines (5)

I believe the answer is:

seams

I'm a little stuck... Click here to teach me more about this clue!

'lines' is the definition.
('seam' can be a synonym of 'line')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for seams that I've seen before include "Joints" , "Strata of minerals" , "Sewn joins" , "Strata of ore or coal suitable for mining" .)

I've seen this clue in the Evening Standard.

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