Mad leader of detachment with grenade goes amok (8)
I believe the answer is:
deranged
'mad' is the definition.
(I've seen this before)
'leader of detachment with grenade goes amok' is the wordplay.
'leader of' says to take the initial letters.
'with' says to put letters next to each other.
'goes amok' is an anagram indicator.
The first letter of 'detachment' is 'd'.
'grenade' is an anagram of 'eranged'.
'd'+'eranged'='DERANGED'
(Other definitions for deranged that I've seen before include "irrational" , "totally without reason" , "Made insane" , "bananas" , "Crazy" .)