Attack a piece of ultra-idealism (4)
I believe the answer is:
raid
'attack' is the definition.
(I know that raid is a type of attack)
'a piece of ultra-idealism' is the wordplay.
'a piece of' says the answer is hidden in the clue.
'RAID' can be found hidden inside 'ultra idealism'.
(Other definitions for raid that I've seen before include "Search by police without warning" , "Assault, attack" , "Sudden short attack" , "Rapid surprise attack" , "Sudden and surprise attack" .)