Seizure in cardiac tissue unsettles surgeon initially (5)
I believe the answer is:
ictus
'seizure' is the definition.
(ictus is a kind of seizure)
'in cardiac tissue unsettles surgeon initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'in cardiac tissue unsettles surgeon' is 'ICTUS'.
(Other definitions for ictus that I've seen before include "Seizure" , "strokes" , "Sudden attack" , "Stroke" , ""fit"" .)