Fool around near pit (5)
I believe the answer is:
abyss
'pit' is the definition.
(I've seen this in another clue)
'fool around near' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'around' means one lot of letters goes inside another.
'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" .)