High point of an architectural plan (3)
I believe the answer is:
alp
'high point' is the definition.
(I've seen this in another clue)
'of an architectural plan' is the wordplay.
'of' says the answer is hidden in the clue.
'ALP' is hidden in the letters of 'architectural plan'.
(Other definitions for alp that I've seen before include "One of a European mountain range" , "Swiss mountain" , "Mont Blanc, for example" , "High mountain" , "high point" .)