Bug fever rebel unfortunately catches (9)
I believe the answer is:
beleaguer
'bug' is the definition.
(synonyms)
'fever rebel unfortunately catches' is the wordplay.
'fever' becomes 'ague' (illness causing fever).
'unfortunately' is an anagram indicator (a bad or unfortunate spelling).
'catches' means one lot of letters goes inside another.
'rebel' anagrammed gives 'beler'.
'ague' going inside 'beler' is 'BELEAGUER'.
(Other definitions for beleaguer that I've seen before include "Put in a very difficult position" , "put in a very difficult situation" , "Surround" .)