Nicaragua guerrilla nursing a fever (4)
I believe the answer is:
ague
'a fever' is the definition.
(I've seen this before)
'nicaragua guerrilla nursing' is the wordplay.
'nursing' says the answer is hidden in the clue (the answer is held or nursed by the clue).
'AGUE' can be found hidden inside 'nicaragua guerrilla'.
(Other definitions for ague that I've seen before include "Feverish fit" , "Malarial fever; chill" , "Feverish illness" , "Old word for fever" , "Hot and cold fever" .)