Turn up guitar on Machine Head as a test (4)
I believe the answer is:
'a test' is the definition.
('exam' can be a synonym of 'test')
'turn up guitar on machine head' is the wordplay.
'turn up' says the letters should be written in reverse.
'guitar' becomes 'axe' (I've seen this before).
'on' means one lot of letters go next to another.
'head' says to take the initial letters.
The first letter of 'machine' is 'm'.
'axe' back-to-front is 'exa'.
'as' acts as a link.
(Other definitions for exam that I've seen before include "Scholastic test" , "Students sit it" , "(Academic) test" , "Formal knowledge test" , "Test (abbr)" .)