Injury resulting from fight with sweetheart (6)
I believe the answer is:
scrape
'injury' is the definition.
(scrape is a kind of injury)
'fight with sweetheart' is the wordplay.
'fight' becomes 'scrap' (a scrap is a minor fight).
'with' says to put letters next to each other.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'scrap'+'e'='SCRAPE'
'resulting from' acts as a link.
(Other definitions for scrape that I've seen before include "Take the surface off" , "Clear or smooth by grating" , "Predicament; obsequious bow" , "fine kettle of fish" , "Graze - awkward predicament" .)