Double agent covers meadow (3)
I believe the answer is:
'meadow' is the definition.
(a lea is a grassy area)
'double agent covers' is the wordplay.
'covers' says the answer is hidden in the clue (I've seen 'covered' mean this (the answer is covered up by the clue)).
'LEA' is hidden within 'double agent'.
(Other definitions for lea that I've seen before include "Open arable land (poet.)" , "Poetically, a piece of meadow or pasture" , "Open area of grassy or arable land" , "Pasture, London river" , "Open pasture" .)