Drink is good, say, regularly (4)
I believe the answer is:
soda
'drink' is the definition.
(soda is a kind of drink)
'is good say regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'is'+'good'+'say'='isgoodsay'
The alternate letters of 'isgoodsay' are 'SODA'.
(Other definitions for soda that I've seen before include "Fizzy drink - chemical compound" , "pressurised liquid" , "In the US, a sweet fizzy drink" , "fizz" , "Whisky mixer" .)