One inside I spot supplying dope (5)
I believe the answer is:
idiot
'dope' is the definition.
(I've seen this before)
'one inside i spot' is the wordplay.
'one' becomes 'i' (Roman numeral).
'inside' means one lot of letters goes inside another.
'spot' becomes 'dot' (I've seen this before).
'i'+'dot'='idot'
'i' put inside 'idot' is 'IDIOT'.
'supplying' acts as a link.
(Other definitions for idiot that I've seen before include "Steal small amount" , "Goose" , "nutter" , "Dolt" , "eg Dostoevski's Myshkin" .)