Nonchalant head of toxicology with unknown victim (8)
I believe the answer is:
casualty
'victim' is the definition.
(casualty is a kind of victim)
'nonchalant head of toxicology with unknown' is the wordplay.
'nonchalant' becomes 'casual' (synonyms).
'head of' suggests taking the first letters.
'with' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
The first letter of 'toxicology' is 't'.
'casual'+'t'+'y'='CASUALTY'
(Other definitions for casualty that I've seen before include "Person killed in accident or war" , "TV hospital series" , "Injured party" , "Something lost or destroyed" , "Victim - injured person" .)