Hostile about Diane, Oscar shows extreme foolishness (6)
I believe the answer is:
idiocy
'extreme foolishness' is the definition.
(I've seen this before)
'hostile about diane oscar' is the wordplay.
'hostile' becomes 'icy' (I've seen this before).
'about' is an insertion indicator.
'diane' becomes 'di' (short for Diane).
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'di'+'o'='dio'
'icy' going around 'dio' is 'IDIOCY'.
'shows' is the link.
(Other definitions for idiocy that I've seen before include "Stupid behaviour or talk" , "Abject stupidity" , "Complete folly" , "Chopper" , "Stupid act" .)