Reason Republican's dismissed fever (4)
I believe the answer is:
ague
'fever' is the definition.
(I've seen this in another clue)
'reason republican's dismissed' is the wordplay.
'reason' becomes 'argue' (synonyms).
'republican' becomes 'r' (abbreviation).
'dismissed' suggests deleting specific letters.
'argue' with 'r' removed is 'AGUE'.
(Other definitions for ague that I've seen before include "Feverish fit" , "Malarial fever or fit of shivering" , "Feverish chill" , "Acute fever; shivering fit" , "Malaria, for example" .)