One wearing a large decorative item (5)
I believe the answer is:
pearl
'decorative item' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'one wearing a large' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'wearing' is an insertion indicator.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'large' becomes 'L' (eg in clothes sizes).
'a' placed inside 'per' is 'pear'.
'pear'+'l'='PEARL'
Can you help me to learn more?
(Other definitions for pearl that I've seen before include "Off-white colour" , "Drop" , "Lustrous structure in a mollusc shell" , "prized example" , "Lustrous structure inside an oyster-shell" .)