Fool around near Gulf (5)
I believe the answer is:
abyss
'gulf' is the definition.
(thesaurus)
'fool around near' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool**).
'around' is an insertion indicator.
'near' becomes 'by' (I've seen this in other clues).
'ass' enclosing 'by' is 'ABYSS'.
(Other definitions for abyss that I've seen before include "Hell; hole" , "Fathomless pit" , "Deep hole, seems bottomless" , "deep depression" , "something rather deep" .)