I love to wear dumb expression (5)
I believe the answer is:
idiom
'expression' is the definition.
(I know that idiom is a type of expression)
'i love to wear dumb' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'to wear' indicates putting letters inside (to wear something is to be covered up by it).
'dumb' becomes 'dim' (synonyms).
'o' inserted within 'dim' is 'diom'.
'i'+'diom'='IDIOM'
(Other definitions for idiom that I've seen before include "Phrase such as over the moon" , "Vocabulary characteristic of a specific group" , "characteristic use of language" , "Language used by group, parlance" , "Phrase with a meaning that is not literal" .)