Mule crossing extremely bumpy canyon (5)
I believe the answer is:
abyss
'canyon' is the definition.
(thesaurus)
'mule crossing extremely bumpy' is the wordplay.
'mule' becomes 'ass' (I can't explain this - if you can you should give a lot more credence to this answer).
'crossing' is an insertion indicator.
'extremely' suggests removing the centre (only the extremes of the word are used).
'bumpy' with its middle taken out is 'by'.
'ass' enclosing 'by' is 'ABYSS'.
Can you help me to learn more?
(Other definitions for abyss that I've seen before include "Bottomless gulf" , "Very deep gorge" , "Deep pit" , "Fathomless pit" , "Measureless chasm" .)