Idiot turned up with what may be Oxford detective (7)
I believe the answer is:
gumshoe
'detective' is the definition.
(gumshoe is a kind of detective)
'idiot turned up with what may be oxford' is the wordplay.
'idiot' becomes 'mug' (a gullible or foolish person).
'turned up' says the letters should be written backwards.
'with' is a charade indicator (letters next to each other).
'what may be oxford' becomes 'shoe' (Oxford is a type of shoe).
'mug' backwards is 'gum'.
'gum'+'shoe'='GUMSHOE'
(Other definitions for gumshoe that I've seen before include "Detective (US colloq.)" , "US detective (informal)" , "Galosh - sleuth" , "One like Sam Spade" , "footwear" .)