Dog, empty, tucks into small piece of cake (5)
I believe the answer is:
wedge
'piece of cake' is the definition.
(I've seen this before)
'dog empty tucks into small' is the wordplay.
'empty' says to hollow out the word (remove centre letters).
'tucks into' is an insertion indicator.
'small' becomes 'wee' (similar in meaning).
'dog' with its middle taken out is 'dg'.
'dg' put into 'wee' is 'WEDGE'.
(Other definitions for wedge that I've seen before include "Force (into narrow space); type of golf club" , "piece of cake" , "Triangular, thick piece" , "Tapering object pushed into a gap - golf club" , "Slice" .)