One gets over wearing stupid expression (5)
I believe the answer is:
idiom
'expression' is the definition.
(idiom is a kind of expression)
'one gets over wearing stupid' is the wordplay.
'one' becomes 'i' (Roman numeral).
'gets over' becomes 'o' (I can't justify this - if you can you should believe this answer much more).
'wearing' is an insertion indicator.
'stupid' becomes 'dim' (I've seen this before).
'o' inserted into 'dim' is 'diom'.
'i'+'diom'='IDIOM'
Can you help me to learn more?
(Other definitions for idiom that I've seen before include "Characteristic style of a period" , "Special linguistic usage" , "Phrase with a meaning that is not literal" , "Vocabulary characteristic of a specific group" , "Style of speaking characteristic of a specific group" .)