One gloomy about ordinary turn of phrase (5)
I believe the answer is:
idiom
'turn of phrase' is the definition.
(I've seen this before)
'one gloomy about ordinary' is the wordplay.
'one' becomes 'i' (Roman numeral).
'gloomy' becomes 'dim'.
'about' is an insertion indicator.
'ordinary' becomes 'O' (abbreviation eg in O-Level, Ordinary Level).
'dim' placed around 'o' is 'diom'.
'i'+'diom'='IDIOM'
(Other definitions for idiom that I've seen before include "Artistic style" , "Linguistic peculiarity" , "Characteristic style of a period" , "Form of linguistic expression" , "Expression that does not make literal sense" .)