Building on the side made princess cross (5)
I believe the answer is:
annex
'building' is the definition.
(I know that annex is a type of building)
'princess cross' is the wordplay.
'princess' becomes 'Anne' (Princess Anne, daughter of Queen Elizabeth).
'cross' becomes 'x' (an X is a cross).
'anne'+'x'='ANNEX'
'on the side made' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for annex that I've seen before include "Append" , "Add to one's territory" , "Addition to a document" , "Add as an extra part" , "An addition that extends a main building" .)