Fodder is turned and left to mature (6)
I believe the answer is:
silage
'fodder' is the definition.
(thesaurus)
'is turned and left to mature' is the wordplay.
'turned' is a reversal indicator.
'and' is a charade indicator (letters next to each other).
'left' becomes 'l' (common abbreviation).
'to mature' becomes 'age' ('age' can be a synonym of 'mature').
'is' in reverse letter order is 'si'.
'si'+'l'+'age'='SILAGE'
(Other definitions for silage that I've seen before include "Preserved fodder for cattle" , "Fodder in store" , "Green fodder stored and fermented" , "Fermented feeding stuff for cattle" , "Animal foodstuff" .)