Wave flag for cake (8)
I believe the answer is:
flapjack
'for cake' is the definition.
(flapjack is a kind of cake)
'wave flag' is the wordplay.
'wave' becomes 'flap' (flap is a kind of wave).
'flag' becomes 'jack' (jack is a kind of flag).
'flap'+'jack'='FLAPJACK'
(Other definitions for flapjack that I've seen before include "One must chew over that" , "Delicacy made with oats and golden syrup" , "Biscuit made with rolled oats" , "Chewy oat biscuit" , "Thick oat biscuit" .)