Fool I spot pinching one (5)
I believe the answer is:
idiot
'fool' is the definition.
(thesaurus)
'i spot pinching one' is the wordplay.
'spot' becomes 'dot' (I've seen this before).
'pinching' means one lot of letters goes inside another (taking inside).
'one' becomes 'i' (Roman numeral).
'dot' enclosing 'i' is 'diot'.
'i'+'diot'='IDIOT'
(Other definitions for idiot that I've seen before include "Dunce" , "Sap" , "Fool; half-wit" , "Stupid person" , "Foolish character" .)