Stamp of excellence (but a blemish on a building?) (8)
I believe the answer is:
hallmark
'stamp of excellence' is the definition.
'hallmark' can be an answer for 'stamp' (I have seen 'Official stamp ' mean 'hallmark' so perhaps 'stamp' could also mean 'hallmark'). I'm not certain of the 'of excellence' bit.
'a blemish on a building?' is the wordplay.
'a blemish' becomes 'mark' (mark is a kind of blemish).
'on' means one lot of letters go next to another.
'a building?' becomes 'hall' (hall is a kind of building).
'mark' after 'hall' is 'HALLMARK'.
'but' is the link.
Can you help me to learn more?
(Other definitions for hallmark that I've seen before include "Assay stamp" , "Distinguishing feature" , "Precious metal stamp" , "Stamp indicating standard of gold etc" , "Assay-stamp" .)