Begin with small cake (5)
I believe the answer is:
start
'begin' is the definition.
(I know that begin can be written as start)
'small cake' is the wordplay.
I cannot really see how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in the answer.
The remaining letters 'tart' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'with' is the link.
Can you help me to learn more?
(Other definitions for start that I've seen before include "Outset" , "Begin with sudden movement of surprise" , "Jump suddenly" , "Onset" , "lead" .)