Test stage (4)
I believe the answer is:
beta
'test stage' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for beta that I've seen before include "Character between alpha and gamma" , "sort of test" , "In a constellation, a star second in brightness" , "second class grade?" , "Greek character - not quite top-notch" .)