I doubt that oaf is about to show up (2,2)
I believe the answer is:
as if
'i doubt that' is the definition.
('as if' may be said to expression disbelief)
'oaf is about to show up' is the wordplay.
'to show' says the answer is hidden in the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'fisa' is hidden within 'oaf is about'.
'fisa' backwards is 'AS IF'.
(Other definitions for as if that I've seen before include "Unbelievable!" , "Or though" , "Yeah right!" , "You're joking!" , "a voice of doubt" .)