I must run after large indicator (6)
I believe the answer is:
litmus
'indicator' is the definition.
(I know that litmus is a type of acid-base indicator)
'i must run after large' is the wordplay.
'run' indicates an anagram.
'after' is a charade indicator (letters next to each other).
'large' becomes 'L' (eg in clothes sizes).
'must' anagrammed gives 'tmus'.
'i'+'tmus'='itmus'
'itmus' put after 'l' is 'LITMUS'.
(Other definitions for litmus that I've seen before include "Old dance" , "It turns red in acid and blue in alkali" , "paper for chemistry test" , "Substance used in test for acidity" , "Acid-alkali testing substance" .)