This — invariably lethal — used for starters in fight? (8)
I believe the answer is:
stiletto
'this invariably lethal used' is the definition.
The definition suggests a singular noun which matches the answer.
'starters in fight?' is the wordplay.
'starters' becomes 'til' (I am not sure about this - if you are sure you should believe this answer much more).
'in' is an insertion indicator.
'fight?' becomes 'setto' (set-to can mean a fight or argument).
'til' inserted into 'setto' is 'STILETTO'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for stiletto that I've seen before include "dangerous weapon" , "High-heeled shoe" , "Slender-bladed dagger - heel" , "Sort of knife" , "Small dagger with a tapered blade" .)