Grassland of Middle Ages (3)
I believe the answer is:
lea
'grassland' is the definition.
(lea is a kind of grassland)
'of middle ages' is the wordplay.
'of' indicates a hidden word.
'LEA' is hidden in the letters of 'middle ages'.
(Other definitions for lea that I've seen before include "Grass area" , "Open area of grassy or arable land" , "''The lowing herd winds slowly o'er the ... '' (Gray)" , "Open grassy land; river" , "Land growing grass" .)