Jump falling short in field (3)
I believe the answer is:
lea
'in field' is the definition.
(I've seen this before)
'jump falling short' is the wordplay.
'jump' becomes 'leap' ('leap' can be a synonym of 'jump').
'falling short' means to remove the last letter.
'leap' with its final letter taken away is 'LEA'.
(Other definitions for lea that I've seen before include "area of grassland" , "''The lowing herd winds slowly o'er the ... '' (Gray)" , "Meadow (poetic)" , "Open grassy land; river" , "Grazing land" .)