Flu or fly (3)
I believe the answer is:
bug
I believe this clue is a double definition.
'flu' is the first definition.
(I've seen this in another clue)
'fly' is the second definition.
(thesaurus)
(Other definitions for bug that I've seen before include "It's snug in a rug (3)" , "Insect - virus" , "An enthusiast" , "Tap" , "Software fault" .)