An indicator master and pupil are involved (6,5)
I believe the answer is:
litmus paper
'an indicator' is the definition.
(litmus paper is a kind of indicator)
'master and pupil are involved' is the wordplay.
'and' is a charade indicator (letters next to each other) (I've seen this in other clues).
'are involved' is an anagram indicator (involved can mean confusing or complex).
'master' put after 'pupil' is 'pupilmaster'.
'pupilmaster' is an anagram of 'LITMUS-PAPER'.
(Other definitions for litmus paper that I've seen before include "Chemical indicator" , "It's used for the acid test" .)