Incomplete study on origin of athletics field (4)
I believe the answer is:
area
'field' is the definition.
(I've seen this before)
'incomplete study on origin of athletics' is the wordplay.
'incomplete' means to remove the last letter.
'study' becomes 'read' ('read' can be a synonym of 'study').
'on' means one lot of letters go next to another.
'origin of' suggests taking the first letters.
The initial letter of 'athletics' is 'a'.
'read' with its final letter removed is 'rea'.
'rea' after 'a' is 'AREA'.
(Other definitions for area that I've seen before include "The measure of a surface" , "Territory" , "Field of activity or surface" , "Succession, order" , "Locality" .)