Fool surrounded by great gap (5)
I believe the answer is:
abyss
'gap' is the definition.
(abyss is a kind of gap)
'fool surrounded by great' is the wordplay.
I cannot quite see how this works, but
'by' is present in the answer.
'fool' could be 'ass' (both can mean a stupid person) and 'ass' is found within the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for abyss that I've seen before include "something rather deep" , "Deep hole, seems bottomless" , "depths of sea" , "Very large and deep chasm" , "Bottomless gulf" .)