Initially loud, into brutalist architecture, then quiet (2,5,5)
I believe the answer is:
at first blush
'initially' is the definition.
(phrase meaning as things appear initially)
'loud into brutalist architecture then quiet' is the wordplay.
I cannot quite understand how this works, but
'loud' could be 'f' (short for forte - 'loud' in music) and 'f' is found within the answer.
an anagram of 'brutalist' is 'atirstblu' which is located in the remaining letters.
'quiet' could be 'sh' (both can be said to tell someone to be quiet) and 'sh' is located in the answer.
No letters remain.
This explanation may well be incorrect...
(Another definition for at first blush that I've seen is " As a first impression".)