Disorder when grenade goes off (7)
I believe the answer is:
derange
'disorder' is the definition.
(I know that derange is a more specific form of the action disorder)
'grenade goes off' is the wordplay.
'goes off' is an anagram indicator.
'grenade' is an anagram of 'DERANGE'.
'when' acts as a link.
(Other definitions for derange that I've seen before include "Throw into disorder" , "Madden" , "Make insane" , "Put into confusion" , "Craze" .)