Nut is article full of goodness (5)
I believe the answer is:
acorn
'nut' is the definition.
(I've seen this in another clue)
'article full of goodness' is the wordplay.
'article' becomes 'an' (indefinite article).
'full of' means one lot of letters goes inside another.
'goodness' becomes 'cor' (both are exclamations of surprise).
'an' placed around 'cor' is 'ACORN'.
'is' is the link.
(Other definitions for acorn that I've seen before include "From which a "tall oak" may grow" , "Baby oak?" , "Fruit of the oak tree" , "Fruit-seed of the oak" , "Source of a great oak" .)