Chump getting past in chasm (5)
I believe the answer is:
abyss
'chasm' is the definition.
(I know that abyss is a type of chasm)
'chump getting past' is the wordplay.
'chump' becomes 'ass' (I've seen this in another clue).
'getting' indicates putting letters inside.
'past' becomes 'by' (as in years gone by).
'ass' going around 'by' is 'ABYSS'.
'in' acts as a link.
(Other definitions for abyss that I've seen before include "depths of sea" , "Deep pit" , "Deep hole, seems bottomless" , "Primal chaos" , "something rather deep" .)