Grass pop artist used regularly (4)
I believe the answer is:
oats
'grass' is the definition.
(I know that oats is a type of cereal)
'pop artist used regularly' is the wordplay.
'used regularly' indicates alternate letters.
'pop'+'artist'='popartist'
The alternate letters of 'popartist' are 'OATS'.
(Other definitions for oats that I've seen before include "A cereal crop" , "Cereal for porridge" , "Food for horses and Scots (said Dr Johnson)" , "Seed grasses" , "Wild ones are sown in youthful excess" .)