Such in infection can ruin a rival (5)
I believe the answer is:
viral
'such in infection can ruin a rival' is the definition.
I can't judge whether this definition defines the answer.
'ruin a rival' is the wordplay.
'ruin' indicates anagramming the letters.
'rival' anagrammed gives 'VIRAL'.
Can you help me to learn more?
(Other definitions for viral that I've seen before include "Caused by pathogenic agent" , "Like some infections" , "Of organisms causing 'flu, etc" , "as an internet sensation" , "Pertaining to an ultramicroscopic infection" .)