With knife at last, girl cut cake (6)
I believe the answer is:
eclair
'cut cake' is the definition.
'eclair' can be an answer for 'cake' (I've seen this before). I'm unsure of the 'cut' bit.
'with knife at last girl' is the wordplay.
'with' means one lot of letters go next to another.
'at last' suggests the final letters.
'girl' becomes 'clair' (**).
The last letter of 'knife' is 'e'.
'e' put next to 'clair' is 'ECLAIR'.
(Other definitions for eclair that I've seen before include "something at teatime?" , "A long thin cake with chocolate covering" , "Food" , "Lacier type of cake" , "Long (but not long-lasting) cake" .)