Centre is replaced on the quiet (2,6)
I believe the answer is:
in secret
'on the quiet' is the definition.
(I've seen this in another clue)
'centre is replaced' is the wordplay.
'replaced' indicates an anagram.
'centre'+'is'='centreis'
'centreis' with letters rearranged gives 'IN SECRET'.
(Other definitions for in secret that I've seen before include "sub rosa" , "privately" , "behind closed doors" , "Confidentially" , "Not openly stated" .)