We’ll drive, finally, over a meadow (3)
I believe the answer is:
lea
'meadow' is the definition.
(a lea is a grassy area)
'we'll drive finally over a' is the wordplay.
'finally' indicates one should take the final letters.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
The final letters of 'well drive' are 'le'.
'le'+'a'='LEA'
(Other definitions for lea that I've seen before include "authority for teachers" , "Open arable land (poet.)" , "Poetically, a piece of meadow or pasture" , "Open country" , "River flowing through London" .)