Tester Rorschach conceals blunder (5)
I believe the answer is:
error
'blunder' is the definition.
(I've seen this before)
'tester rorschach conceals' is the wordplay.
'conceals' indicates the answer is hidden within the clue.
'ERROR' can be found hidden inside 'tester rorschach'.
(Other definitions for error that I've seen before include "Miscalculation" , "Slip-up" , "Trial and ___" , "Bug" , "Mistake from terrorist" .)