Deaden live heart, arresting metabolism initially (6)
I believe the answer is:
benumb
'deaden' is the definition.
(I've seen this in another clue)
'live heart arresting metabolism initially' is the wordplay.
'live' becomes 'be' (synonyms).
'heart' becomes 'nub' (synonyms).
'arresting' indicates putting letters inside.
'initially' says to take the initial letters.
The first letter of 'metabolism' is 'm'.
'nub' enclosing 'm' is 'numb'.
'be'+'numb'='BENUMB'
(Other definitions for benumb that I've seen before include "Stun" , "Deprive of feeling" , "Make dazed or unfeeling" , "Maybe anaesthetise" .)