Begin a small cake (5)
I believe the answer is:
start
'begin' is the definition.
(I know that start is a more specific form of begin)
'a small cake' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for start that I've seen before include "Begin; flinch" , "Onset" , "Jerk" , "get under way" , "Begin - opening move" .)