Presence of virus; confine it somehow (9)
I believe the answer is:
infection
'presence of virus' is the definition.
'confine it somehow' is the wordplay.
'somehow' indicates an anagram.
'confine'+'it'='confineit'
'confineit' with letters rearranged gives 'INFECTION'.
(Other definitions for infection that I've seen before include "Contamination by micro-organism" , "what may be caught" , "Invasion of the body by pathogenic micro-organisms" , "Process of catching virus" , "Disease caught" .)