Normal diary Ron replaced (8)
I believe the answer is:
ordinary
'normal' is the definition.
(I've seen this before)
'diary ron replaced' is the wordplay.
'replaced' is an anagram indicator.
'diary'+'ron'='diaryron'
'diaryron' with letters rearranged gives 'ORDINARY'.
(Other definitions for ordinary that I've seen before include "Unremarkable" , "charge for arms" , "Run of the mill" , "Undistinguished" , "Common-or-garden" .)