Dubious expression regularly mars riffs (2,2)
I believe the answer is:
as if
'dubious expression' is the definition.
'as if' can be an answer for 'expression' (I have seen 'Expression of incredulity' mean 'as if' so perhaps 'expression' could also mean 'as if'). I am not sure about the 'dubious' bit.
'regularly mars riffs' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'mars'+'riffs'='marsriffs'
The alternate letters of 'marsriffs' are 'AS-IF'.
Can you help me to learn more?
(Other definitions for as if that I've seen before include "Unbelievable!" , "I very much doubt it!" , "Or though" , "Seemingly" , "Yeah right!" .)