Expression of incredulity seen originally in a poem (2,2)
I believe the answer is:
as if
'expression of incredulity' is the definition.
(I've seen this before)
'seen originally in a poem' is the wordplay.
'originally' says to take the initial letters.
'in' indicates putting letters inside.
'poem' becomes 'if' (I've seen this in other clues**).
The initial letter of 'seen' is 's'.
'a'+'if'='aif'
's' placed into 'aif' is 'AS IF'.
(Other definitions for as if that I've seen before include "Seemingly" , "You're joking!" , "a voice of doubt" , "Expression of incredulity" , "Unbelievable!" .)