Prompt pupil concealed some evidence (4)
I believe the answer is:
clue
'some evidence' is the definition.
(clue is a kind of evidence)
'prompt pupil concealed' is the wordplay.
'prompt' becomes 'cue' (cue can mean a prompt or signal).
'pupil' becomes 'L' (as in a learner driver with L-plates on their car).
'concealed' means one lot of letters goes inside another.
'cue' placed around 'l' is 'CLUE'.
(Other definitions for clue that I've seen before include "Part of a crossword" , "one of 28 here" , "Pointer" , "Crossword item" , "Crossword help" .)