I don't believe that last gift's totally unwrapped (2,2)
I believe the answer is:
as if
'i don't believe that' is the definition.
('as if' is a response to something unbelievable)
'last gift's totally unwrapped' is the wordplay.
I cannot quite see how this works, but
'last' could be 'f' (I've seen this in other clues) and 'f' is found in the answer.
The remaining letters 'asi' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for as if that I've seen before include "I very much doubt it!" , "Expression of incredulity" , "Unbelievable!" , "You're joking!" , "highly unlikely" .)