Turned over great herbs I mixed? Nonsense! (9)
I believe the answer is:
gibberish
'nonsense' is the definition.
(gibberish is a kind of nonsense)
'turned over great herbs i mixed?' is the wordplay.
'turned over' is a reversal indicator.
'great' becomes 'big' ('big' can be a synonym of 'great'**).
'mixed?' indicates an anagram (I've seen 'mixed' mean this).
'big' back-to-front is 'gib'.
'herbs'+'i'='herbsi'
'herbsi' is an anagram of 'berish'.
'gib'+'berish'='GIBBERISH'
(Other definitions for gibberish that I've seen before include "Rubbish" , "Tosh" , "rhubarb" , "moonshine" , "Nonsensical speech" .)