Field: space confining last of oxen (5)
I believe the answer is:
arena
'field' is the definition.
('arena' can be a synonym of 'field')
'space confining last of oxen' is the wordplay.
'space' becomes 'area' (I've seen this before**).
'confining' is an insertion indicator.
'last of' says to take the final letters.
The last letter of 'oxen' is 'n'.
'area' placed around 'n' is 'ARENA'.
(Other definitions for arena that I've seen before include "Sporting enclosure" , "An era (anag.)" , "Cheap Prom tickets" , "Place of activity, especially conflict" , "Part of a stadium" .)