The fool goes around by deep gorge (5)
I believe the answer is:
abyss
'deep gorge' is the definition.
(I've seen this in another clue)
'the fool goes around by' is the wordplay.
'the fool' becomes 'ass' (both can mean a stupid person).
'goes around' means one lot of letters goes inside another.
'ass' enclosing 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "Deep hole, seems bottomless" , "Measureless chasm" , "great depth" , "Deep gulf" , "Seemingly bottomless chasm" .)