Fool I spot holding one (5)
I believe the answer is:
idiot
'fool' is the definition.
(thesaurus)
'i spot holding one' is the wordplay.
'spot' becomes 'dot' (I've seen this before).
'holding' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'dot' going around 'i' is 'diot'.
'i'+'diot'='IDIOT'
(Other definitions for idiot that I've seen before include "Dolt" , "Dostoevsky hero" , "Fool, silly person" , "Sap" , "Dunce" .)