Line going up, boundary always vertical (1-4)
I believe the answer is:
y-axis
'vertical' is the definition.
(I've seen this before)
'line going up boundary always' is the wordplay.
I cannot quite see how this works, but
'line' could be 'axis' (axis is a kind of line) and 'axis' is found in the answer.
A single letter 'y' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for y-axis that I've seen is " Vertical line on a graph".)