Six registered American infective agent (5)
I believe the answer is:
virus
'infective agent' is the definition.
'virus' can be an answer for 'agent' (I have seen 'Infectious agent' mean 'virus' so perhaps 'agent' could also mean 'virus'). I am not sure about the 'infective' bit.
'six registered american' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'registered' becomes 'R' (abbreviation, as in registered trademark).
'american' becomes 'US' (United States).
'vi'+'r'+'us'='VIRUS'
Can you help me to learn more?
(Other definitions for virus that I've seen before include "Harmful agent to animals and computers" , "Hidden destructive code in a computer program, or cause of illness" , "Corrupting influence" , "Microscopic entity capable of reproducing within cells" , "Hidden destructive code in computer program - makes you sick" .)