Expression I had in mind initially about love (5)
I believe the answer is:
idiom
'expression' is the definition.
(I know that idiom is a type of expression)
'i had in mind initially about love' is the wordplay.
'i had' becomes 'i'd' (I'd).
'initially' says to take the initial letters.
'about' means one lot of letters goes inside another.
'love' becomes 'o' (love means zero in tennis).
The initial letters of 'in mind' is 'im'.
'im' placed around 'o' is 'iom'.
'id'+'iom'='IDIOM'
(Other definitions for idiom that I've seen before include "Colloquial saying" , "characteristic use of language" , "Language peculiar to a particular group" , "Special linguistic usage" , "Phrase with a meaning that is not literal" .)