This — invariably lethal — used for starters in fight? (8)

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

&lit. Initial letters of " t his i nvariably l ethal" in set-to. My only problem with the clue is that a stiletto, whilst lethal, wouldn't be invariably lethal. Maybe the question mark covers both dubious assertions (that it is always lethal and that it is used at the start of a fight).