Some pop a question that's hard to understand (6)
I believe the answer is:
opaque
'hard to understand' is the definition.
(I've seen this before)
'some pop a question' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'OPAQUE' is hidden within 'pop a question'.
'that's' acts as a link.
(Other definitions for opaque that I've seen before include "Lacking transparency" , "Nebulous, unclear" , "Puzzling - dull" , "Impenetrable to light" , "Dark" .)