Acid or screen follower (4)
I believe the answer is:
test
'acid or screen follower' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for test that I've seen before include "Trial; Hampshire river" , "Short exam" , "Trial; cricket match" , "Trial - international game" , "Critical examination or trial of qualities or abilities" .)