Take a chance with article under test (4)
I believe the answer is:
beta
'test' is the definition.
(beta test is a kind of test)
'take a chance with article under' is the wordplay.
'take a chance with' becomes 'bet'.
'article' becomes 'a' (indefinite article).
'under' is a charade indicator (letters next to each other).
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "2nd letter of Greek alphabet" , "Letter before gamma" , "Second-rate" , "sort of test" , "Character between alpha and gamma" .)