Fever upset the Ugandans to some extent (4)
I believe the answer is:
ague
'fever' is the definition.
(I've seen this before)
'upset the ugandans to some extent' is the wordplay.
'upset' shows that the letters should be reversed in order.
'to some extent' says the answer is hidden in the clue.
'euga' is hidden in the letters of 'the ugandans'.
'euga' written backwards gives 'AGUE'.
(Other definitions for ague that I've seen before include "Complaint" , "Sickness" , "Burning fever" , "met with cheek in Twelfth Night" , "Acute fever; shivering fit" .)