In the field, it's a credit and an eye-opener (4)
I believe the answer is:
'in the field' is the definition.
I can't tell whether this definition defines the answer.
'a credit and an eye-opener' is the wordplay.
'credit' becomes 'cr'.
'and' means one lot of letters go next to another.
'opener' suggests taking the first letters.
The initial letter of 'eye' is 'e'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for acre that I've seen before include "Unit of land area" , "0.405 of a hectare" , "Israeli seaport" , "Ancient city in Israel" , "Area measure - Israeli port - Brazilian state" .)