Rough decision: head of Whipsnade must leave (3)
I believe the answer is:
ill
'rough' is the definition.
(rough can mean unwell)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ill that I've seen before include "Groggy" , "Ailing" , "Badly - in need of treatment" , "Infirm" , "Unwell from bacillus" .)