Firm that is checking approval for online tracking code (6)
I believe the answer is:
cookie
'online tracking code' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'firm that is checking approval' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'checking' is an insertion indicator (check can mean to control or arrest).
'approval' becomes 'ok' (OK is an example).
'co'+'ie'='coie'
'coie' enclosing 'ok' is 'COOKIE'.
'for' is the link.
Can you help me to learn more?
(Other definitions for cookie that I've seen before include "Biscuit; computer data" , "An American sweet biscuit" , "US bicky" , "Identifying data; biscuit" , "Computer tracker; US biscuit" .)