In navel diamond perhaps falling (8)
I believe the answer is:
tumbling
'falling' is the definition.
(tumbling is a kind of falling)
'in navel diamond' is the wordplay.
I cannot really see how this works, but
'in' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'perhaps' acts as a link.
Can you help me to learn more?
(Other definitions for tumbling that I've seen before include "Falling headlong" , "Quickly declining" , "Acrobatics" , "Rolling over and over, as a gymnast might" , "Falling down, as if collapsing" .)