Indicator illuminated, has to set off (6,4)
I believe the answer is:
litmus test
'indicator' is the definition.
(I know that litmus test is a type of acid-base indicator)
'illuminated has to set off' is the wordplay.
'illuminated' becomes 'lit' (synonyms).
'has to' becomes 'must' (I've seen this in another clue).
'off' indicates anagramming the letters.
'set' with letters rearranged gives 'est'.
'lit'+'must'+'est'='LITMUS TEST'
(Other definitions for litmus test that I've seen before include "Chemical trial" , "Observer may see red after one" .)