Never mind causing death -- hospital overlooked one (3,5)
I believe the answer is:
let alone
'never mind' is the definition.
(I've seen this before)
'causing death hospital overlooked one' is the wordplay.
'causing death' becomes 'lethal' (I've seen this before).
'hospital' becomes 'H' (abbreviation for hospital on maps).
'overlooked' suggests deleting specific letters.
'lethal' with 'h' taken away is 'letal'.
'letal'+'one'='LET-ALONE'
(Other definitions for let alone that I've seen before include "Don't bother someone - allow to be solitary" , "Don't touch" , "Not to mention - much less - avoid" , "Not to mention (thing)" , "Not to mention, never mind" .)