Fool around near pit (5)
I believe the answer is:
abyss
'pit' is the definition.
(I've seen this before)
'fool around near' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'around' indicates putting letters inside.
'near' becomes 'by' (near means close by).
'ass' going around 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "gaping hole" , "Very large and deep chasm" , "Deep pit" , "Measureless chasm" , "Hell; hole" .)