There's nothing in one stupid phrase (5)
I believe the answer is:
idiom
'phrase' is the definition.
(I have seen 'particular phrase' mean 'idiom' so perhaps 'phrase' could also mean 'idiom')
'there's nothing in one stupid' is the wordplay.
'there's nothing' becomes 'o' (I've seen this in other clues).
'in' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'stupid' becomes 'dim' (I've seen this before).
'i'+'dim'='idim'
'o' placed into 'idim' is 'IDIOM'.
(Other definitions for idiom that I've seen before include "Word usage peculiar to a specific group" , "Language peculiar to a particular group" , "Colloquial saying" , "Expression that's characteristic of particular group" , "Language used by group, parlance" .)