Abstainer to tell eater off (11)
I believe the answer is:
teetotaller
'abstainer' is the definition.
(teetotaller is a kind of abstainer)
'to tell eater off' is the wordplay.
'off' indicates anagramming the letters.
'to'+'tell'+'eater'='totelleater'
'totelleater' with letters rearranged gives 'TEETOTALLER'.
(Other definitions for teetotaller that I've seen before include "Non-drinker" , "Abstainer from drink" , "Abstainer from alcohol" , "no supper!" , "Rechabite" .)