Fool gets by in gulf (5)
I believe the answer is:
abyss
'gulf' is the definition.
(an abyss is a gulf or chasm)
'fool gets by' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'gets' is an insertion indicator (gets can mean captures or absorbs).
'ass' placed around 'by' is 'ABYSS'.
'in' is the link.
(Other definitions for abyss that I've seen before include "Hell; hole" , "Deep pit" , "Measureless chasm" , "deep depression" , "Very large and deep chasm" .)