Triad (5)
I believe the answer is:
three
'triad' is the definition.
(I know that triad can be written as three)
This is the entire clue.
(Other definitions for three that I've seen before include "Number of blind mice" , "Small number that still makes a crowd?" , "There (anag.)" , "One, one, one" , "Start to count back, briefly" .)