Only even prime number (3)
I believe the answer is:
two
'only even prime number' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for two that I've seen before include "Number in company, some say" , "eg Twins" , "Minimal number of partners" , "A low number" , "Number of Gentlemen of Verona" .)