A riddle lacking conclusion -- I doubt that! (2,2)
I believe the answer is:
as if
'i doubt that' is the definition.
('as if' may be said to expression disbelief)
'a riddle lacking' is the wordplay.
'riddle' becomes 'sift' (riddle can mean to sieve or sift).
'lacking' means to remove the last letter (the word is incomplete or lacking).
'sift' with its last letter removed is 'sif'.
'a'+'sif'='AS-IF'
'conclusion' acts as a link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for as if that I've seen before include "Or though" , "a voice of doubt" , "Unbelievable!" , "You're joking!" , "Seemingly" .)