Treated a mowed field (6)
I believe the answer is:
meadow
'field' is the definition.
(a meadow is a field)
'treated a mowed' is the wordplay.
'treated' is an anagram indicator.
'a'+'mowed'='amowed'
'amowed' with letters rearranged gives 'MEADOW'.
(Other definitions for meadow that I've seen before include "Tract of cultivated grassland" , "Piece of grassland" , "Grassy field" , "Seventeen dn used for hay" , "Field used for hay" .)