House in architect's drawing, silver in principle (11)
I believe the answer is:
plantagenet
'house' is the definition.
(historical royal house)
'architect's drawing silver in principle' is the wordplay.
'architect's drawing' becomes 'plan' (I've seen this in another clue).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'in' means one lot of letters goes inside another.
'principle' becomes 'tenet' (a tenet is a principle or value).
'ag' going inside 'tenet' is 'tagenet'.
'plan'+'tagenet'='PLANTAGENET'
'in' acts as a link.
(Other definitions for plantagenet that I've seen before include "Former English royal family" , "royal dynasty" , "Member of English royal house from Henry II to Richard III" , "9 [HOUSE]" .)