First odd prime (5)
I believe the answer is:
three
'first odd prime' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for three that I've seen before include "Square root of nine" , "Number of Dumas's musketeers" , "Trinity, triad" , "A crowd, you might say" , "There (anag.)" .)