Infection starting very innocently rocks us seriously (5)
I believe the answer is:
virus
'infection' is the definition.
(thesaurus)
'starting very innocently rocks us seriously' is the wordplay.
'starting' indicates taking the first letters.
The initial letters of 'very innocently rocks us seriously' is 'VIRUS'.
(Other definitions for virus that I've seen before include "Malicious code" , "What may kill your computer" , "Cause of computer malfunctions?" , "Particle injurious to people (or computers?)" , "Hidden destructive code in computer program - makes you sick" .)