It’s not noted for perpendicular architecture (4)
I believe the answer is:
pisa
'perpendicular architecture' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'it's not noted' 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 see.
This explanation may well be incorrect...
'for' is the link.
Can you help me to learn more?
(Other definitions for pisa that I've seen before include "Italian city with famous tower" , "There's a marked inclination here" , "City in Tuscany" , "Site of the famous leaning tower" , "City on the River Arno" .)