I spot one inside supplying dope (5)
I believe the answer is:
idiot
'dope' is the definition.
(I've seen this before)
'i spot one inside' is the wordplay.
'spot' becomes 'dot' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'inside' means one lot of letters goes inside another.
'dot' enclosing 'i' is 'diot'.
'i'+'diot'='IDIOT'
'supplying' acts as a link.
(Other definitions for idiot that I've seen before include "Ignoramus's" , "eg Dostoevski's Myshkin" , "Foolish character" , "Fool, silly person" , "Charlie" .)