Turn of phrase I love about Diane and Mike (5)
I believe the answer is:
idiom
'turn of phrase' is the definition.
(I've seen this before)
'i love about diane and mike' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'about' means one lot of letters go next to another.
'diane' becomes 'di' (short for Diane).
'and' says to put letters next to each other.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'o' after 'di' is 'dio'.
'i'+'dio'+'m'='IDIOM'
(Other definitions for idiom that I've seen before include "Linguistic peculiarity" , "Manner of speaking or writing" , "Form of linguistic expression" , "Language used by group, parlance" , "Distinctive expression" .)