It certainly makes you wonder! (6)
I believe the answer is:
'wonder' is the definition.
(I know that wonder can be written as marvel)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for marvel that I've seen before include "Express astonishment" , "Be filled with surprise or wonder" , "Wonder, be astonished" , "Amazing person or event" , "Be filled with wonder - something wondrous" .)