Goodness found in an oak seed (5)
I believe the answer is:
acorn
'oak seed' is the definition.
(I've seen this before)
'goodness found in an' is the wordplay.
'goodness' becomes 'cor' (both are exclamations of surprise).
'found in' indicates putting letters inside.
'cor' put within 'an' is 'ACORN'.
(Other definitions for acorn that I've seen before include "cup's content" , "Baby oak?" , "Tree seed" , "Source of a great oak" , "Might source" .)