Acted out of misguided idealism (3)
I believe the answer is:
did
'acted' is the definition.
(to do is to perform a particular action)
'of misguided idealism' is the wordplay.
'of' indicates a hidden word.
'DID' is hidden in the letters of 'misguided idealism'.
'out' acts as a link.
(Other definitions for did that I've seen before include "Carried out an act" , "Swindled" , "was satisfactory" , "Carried out, performed" , "Performed an action" .)