Fool about near chasm (5)
I believe the answer is:
abyss
'chasm' is the definition.
(abyss is a kind of chasm)
'fool about near' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool).
'about' indicates putting letters inside.
'near' becomes 'by' (I've seen this in other clues).
'ass' placed around 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "crevasse" , "great depth" , "something rather deep" , "canyon?" , "Seemingly bottomless chasm" .)