In the field, it's a credit and an eye-opener (4)
I believe the answer is:
acre
'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'.
'a'+'cr'+'e'='ACRE'
'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" .)