Prize article mostly secure in box, perhaps (8)
I believe the answer is:
treasure
'prize' is the definition.
(synonyms)
'article mostly secure in box perhaps' is the wordplay.
'article' becomes 'a' (example of the part-of-speech).
'mostly secure' becomes 'sur' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'box perhaps' becomes 'tree' (box tree is a type of tree).
'a'+'sur'='asur'
'asur' going within 'tree' is 'TREASURE'.
Can you help me to learn more?
(Other definitions for treasure that I've seen before include "someone valued" , "Highly-prize a possession" , "Hold dearly, like buried gold etc" , "Highly-valued possession" , "Robert Louis Stevenson novel" .)