A piece of cake became a symbol, to some extent (4)
I believe the answer is:
easy
'a piece of cake' is the definition.
(I've seen this before)
'became a symbol to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'EASY' is hidden in the letters of 'became a symbol'.
(Other definitions for easy that I've seen before include "Simply achieved" , "Convenient" , "Straightforward" , "Effortless, uncomplicated" , "Undemanding" .)