The bird Peter stupidly left behind (6)
I believe the answer is:
petrel
'the bird' is the definition.
(petrel is a kind of bird)
'peter stupidly left behind' is the wordplay.
'stupidly' is an anagram indicator (a stupid ordering of the letters).
'left' becomes 'l' (common abbreviation).
'behind' is a charade indicator (letters next to each other).
'peter' with letters rearranged gives 'petre'.
'petre'+'l'='PETREL'
(Other definitions for petrel that I've seen before include "Seabird sounds like fuel" , "Mother Carey's chicken" , "Bird that sounds like fuel" , "The sea-bird known as stormy" , "18's [BIRD]" .)