A homeless virus possibly the ideal (10)
I believe the answer is:
perfection
'the ideal' is the definition.
(perfection is a kind of ideal)
'a homeless virus' is the wordplay.
I cannot really understand how this works, but
'a' could be 'per' and 'per' is found in the answer.
This explanation may well be incorrect...
'possibly' is the link.
(Other definitions for perfection that I've seen before include "Highest state" , "Faultlessness" .)