Endless charm shown by heavy drinker, indicator of what comes next (9)
I believe the answer is:
harbinger
'indicator of what comes next' is the definition.
(a harbinger signals the arrival of something else)
'endless charm shown by heavy drinker' is the wordplay.
'endless' means to look at the middle letters.
'shown by' says to put letters next to each other.
'heavy drinker' becomes 'binger' (someone who binges).
The middle letters of 'charm' are 'har'.
'har'+'binger'='HARBINGER'
(Other definitions for harbinger that I've seen before include "Precursor" , "Herald - forerunner" , "It indicates the future" , "One giving advance warning" , "Early sign" .)