It's not noted for its perpendicular architecture (4)
I believe the answer is:
pisa
'perpendicular architecture' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'it's not noted for its' is the wordplay.
I cannot really see how this works, but
'it' could be 'sa' (old-fashioned abbreviation for sex appeal) and 'sa' is present in the answer.
The remaining letters 'pi' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for pisa that I've seen before include "City in Tuscany" , "Site of the famous leaning tower" , "Location of leaning tower" , "Italian city with famous tower" , "Tuscan city" .)