Part of shoe gashed - that's a blow (8)
I believe the answer is:
uppercut
'a blow' is the definition.
(uppercut is a kind of blow)
'part of shoe gashed' is the wordplay.
'part of shoe' becomes 'upper' (I've seen this before).
'gashed' becomes 'cut' ('cut' can be a synonym of 'gashed').
'upper'+'cut'='UPPERCUT'
'that's' acts as a link.
(Other definitions for uppercut that I've seen before include "Boxing punch delivered vertically" , "Blow from fist to chin" , "Punch to opponent's chin from below" , "Swinging blow in boxing" , "Swinging blow from below aimed at an opponent's chin" .)