Stress I can take unless stressed initially (5)
I believe the answer is:
ictus
'stress' is the definition.
(I've seen this before)
'i can take unless stressed initially' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'can take unless stressed' is 'ctus'.
'i'+'ctus'='ICTUS'
(Other definitions for ictus that I've seen before include "Seizure" , "Metrical stress" , "Sudden attack" , "Is cut (anag.)" , "strokes" .)