Perrier has been sampled regularly before, once (3)
I believe the answer is:
ere
'before once' is the definition.
(archaic word for 'before')
'perrier has been sampled regularly' is the wordplay.
'has been sampled regularly' indicates alternate letters.
The alternating letters of 'perrier' are 'ERE'.
(Other definitions for ere that I've seen before include "Archaic word for before - a palindrome" , "Before (lit.)" , "Before (poetic)" , "Before, poetically" , "Old word for before" .)