Lunatic called into action (8)
I believe the answer is:
deranged
'lunatic' is the definition.
(thesaurus)
'called into action' is the wordplay.
'called' becomes 'rang' ('ring' can be a synonym of 'call').
'into' is an insertion indicator.
'action' becomes 'deed' (deed is a kind of action).
'rang' inserted into 'deed' is 'DERANGED'.
(Other definitions for deranged that I've seen before include "Crazy" , "Mentally unbalanced" , "What Ophelia was eventually" , "totally without reason" , "Crackpot" .)