Second coffee and third piece of tart (6)
I believe the answer is:
latter
'second' is the definition.
(latter can mean the second of two mentioned things)
'coffee and third piece of tart' is the wordplay.
I cannot really understand how this works, but
'coffee' could be 'latte' (type of coffee drink) and 'latte' is present in the answer.
'piece' could be 'r' (abbreviation for Rook) and 'r' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for latter that I've seen before include "Second-mentioned of two" , "Nearer the end" , "second one" , "Second-mentioned; recent" , "Last of those mentioned" .)