Eager to know everything corny then? (3,4)
I believe the answer is:
all ears
'eager to know' is the definition.
'everything corny then?' is the wordplay.
'everything' becomes 'all' (I've seen this before).
'corny then?' becomes 'ears' (I can't explain this - if you can you should believe this answer much more).
'all'+'ears'='ALL EARS'
(Other definitions for all ears that I've seen before include "Prepared to listen" , "Very alert" , "Listening attentively" , ".Listening very intently" , "Being very attentive" .)