Unreliable force making wild assaults after losing leader (8)
I believe the answer is:
feckless
'unreliable' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for feckless that I've seen before include "Irresponsible (with money)" , "Helpless or irresponsible" , "Feeble - ineffectual" , "Incompetent, feeble" , "Not fit for responsibility" .)