Ignored by superior? That’s a blow (8)
I believe the answer is:
uppercut
'a blow' is the definition.
(uppercut is a kind of blow)
'ignored by superior?' is the wordplay.
'ignored' becomes 'cut' (I've seen this in other clues).
'by' says to put letters next to each other.
'superior?' becomes 'upper' (I've seen this before**).
'cut' put after 'upper' is 'UPPERCUT'.
'that's' acts as a link.
(Other definitions for uppercut that I've seen before include "Punch to opponent's chin from below" , "Swinging blow in boxing" , "Boxer's blow" , "Blow from fist to chin" , "sock" .)