Knife fight about it upset learner driver (8)
I believe the answer is:
stiletto
'knife' is the definition.
(stiletto is a kind of knife)
'fight about it upset learner driver' is the wordplay.
'fight' becomes 'set-to' (set-to can mean a fight or argument).
'about it' means one lot of letters goes inside another (some letters go around or about others).
'upset learner driver' becomes 'til' (I can't explain this - if you can you should believe this answer much more).
'setto' placed around 'til' is 'STILETTO'.
Can you help me to learn more?
(Other definitions for stiletto that I've seen before include "Small dagger and high narrow heel" , "Dagger with narrow blade, or a high heel" , "Narrow-bladed dagger" , "Kind of heel" , "Bedspread" .)