Coffee -- the second of two unfinished (5)
I believe the answer is:
'coffee' is the definition.
(I've seen this before)
'the second of two unfinished' is the wordplay.
'the second of two' becomes 'latter' (I've seen this before).
'unfinished' means to remove the last letter.
'latter' with its last letter taken off is 'LATTE'.
(Other definitions for latte that I've seen before include "See 21" , "Milky coffee drink" , "Coffee beverage" , "Coffee served with hot milk" , "Espresso added to frothed steamed milk" .)