Quiet order became smaller (6)
I believe the answer is:
shrank
'became smaller' is the definition.
(I've seen this before)
'quiet order' is the wordplay.
'quiet' becomes 'sh'.
'order' becomes 'rank' (rank order is a kind of order).
'sh'+'rank'='SHRANK'
(Other definitions for shrank that I've seen before include "Contracted, grew smaller" , "Reduced in size" , "Got less" , "Became smaller" , "Shrivelled" .)