I love wearing stupid expression (5)
I believe the answer is:
idiom
'expression' is the definition.
(I know that idiom is a type of expression)
'i love wearing stupid' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'wearing' indicates putting letters inside.
'stupid' becomes 'dim' (similar in meaning).
'o' going into 'dim' is 'diom'.
'i'+'diom'='IDIOM'
(Other definitions for idiom that I've seen before include "Use of words" , "Form of linguistic expression" , "Form or variety of language" , "Distinctive expression" , "Word usage peculiar to a specific group" .)