Risk of heart malfunctioning after fifties, essentially (6)
I believe the answer is:
threat
'risk' is the definition.
(I've seen this before)
'heart malfunctioning after fifties essentially' is the wordplay.
'malfunctioning' indicates an anagram.
'after' says to put letters next to each other.
'essentially' means to look at the middle letters.
The central letter of 'fifties' is 't'.
'heart' with letters rearranged gives 'threa'.
'threa'+'t'='THREAT'
'of' acts as a link.
(Other definitions for threat that I've seen before include "Statement of harmful intent" , "Menace of impending evil" , "Declaration of intention to harm another" , "Potential source of danger" , "An intimidating warning" .)