Foolish person crossing through deep chasm (5)
I believe the answer is:
abyss
'deep chasm' is the definition.
(I've seen this before)
'foolish person crossing through' is the wordplay.
'foolish person' becomes 'ass' (I've seen this before).
'crossing' means one lot of letters goes inside another.
'through' becomes 'by'.
'ass' going around 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "Seemingly bottomless chasm" , "ocean depth" , "canyon?" , "depths of sea" , "Hell; hole" .)