The field of simple arithmetic (3)
I believe the answer is:
lea
'the field' is the definition.
(lea can mean a meadow or field of grass)
'of simple arithmetic' is the wordplay.
'of' indicates a hidden word.
'LEA' is hidden within 'simple arithmetic'.
(Other definitions for lea that I've seen before include "Grass area" , "Open area of grassy or arable land" , "Tract of pastureland" , "Meadow (poetic)" , "Open grassy land; river" .)