Test 50 per cent of people as a warning (7)
I believe the answer is:
example
'a warning' is the definition.
(example is a kind of warning)
'test 50 per cent of people' is the wordplay.
'test' becomes 'exam' ('exam' can be a synonym of 'test').
'50 per cent of' indicates taking half.
'people' with half the letters taken is 'ple'.
'exam'+'ple'='EXAMPLE'
'as' acts as a link.
(Other definitions for example that I've seen before include "lesson" , "Characteristic item" , "not the actual version" , "Thing illustrating a general rule" , "Something to be imitated" .)