Defect after leader's been taken ill (6)
I believe the answer is:
ailing
'ill' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ailing that I've seen before include "Ill" , "Sickly, poorly" , "Infertile" , "Suffering" , "feeling sick" .)