Not the architectural style associated with Pisa's tower? (13)
I believe the answer is:
perpendicular
'tower?' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for perpendicular that I've seen before include "Sort of architecture" , "style of construction" , "At ninety degrees" , "Gothic style following 7" , "Vertical - at 90 degrees" .)