Speculation linked to a test (4)
I believe the answer is:
beta
'test' is the definition.
(beta test is a kind of test)
'speculation linked to a' is the wordplay.
'speculation' becomes 'bet' (bet is a kind of speculation).
'linked to' is a charade indicator (letters next to each other).
'bet'+'a'='BETA'
(Other definitions for beta that I've seen before include "Second-class" , "It's next after alpha" , "Old character" , "software under test" , "In a constellation, a star second in brightness" .)