Test warnings? Not half! (4)
I believe the answer is:
exam
'test' is the definition.
('exam' can be a synonym of 'test')
'warnings? not half' is the wordplay.
'warnings?' becomes 'examples' (example is a kind of warning).
'not half' suggests halving the letters.
'examples' cut in half is 'EXAM'.
(Other definitions for exam that I've seen before include "Scholastic test" , "Written test" , "Oral maybe" , "Important test" , "paper" .)