Are about a hundred in the field (4)
I believe the answer is:
acre
'field' is the definition.
(I've seen this in another clue)
'are about a hundred' is the wordplay.
'about' is an insertion indicator.
'a hundred' becomes 'C' (Roman numerals).
'are' going around 'c' is 'ACRE'.
'in the' is the link.
(Other definitions for acre that I've seen before include "4840 sq yds" , "stretch of land" , "holy city" , "Unit of area - Israeli port" , "Four thousand, eight hundred and forty square yards" .)