Potential killer, European breaking measure of work up (7)
I believe the answer is:
grenade
'potential killer' is the definition.
(grenades may kill people)
'european breaking measure of work up' is the wordplay.
'european' becomes 'Dane' (someone from Denmark).
'breaking' indicates putting letters inside.
'measure of work' becomes 'erg' (unit of work or energy in physics).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'dane' put within 'erg' is 'edanerg'.
'edanerg' in reverse letter order is 'GRENADE'.
(Other definitions for grenade that I've seen before include "Small explosive that is thrown" , "Handy bomb" , "Hand-thrown explosive" , "Small explosive device" , "Bomb with a pin" .)