Unreliable force making wild assaults after losing leader (8)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Unreliable force making wild assaults after losing leader F (=force, in physics) + <r> ECKLESS (=making wild assaults; “after losing leader” means first letter is dropped); feckless is helpless, feeble, hence “unreliable”