Part blew over backwards, I say (5)
I believe the answer is:
vowel
'i say' is the definition.
(I is a vowel)
'part blew over backwards' is the wordplay.
'part' indicates the answer is hidden within the clue.
'backwards' says the letters should be written backwards.
'lewov' is hidden within 'blew over'.
'lewov' written backwards gives 'VOWEL'.
(Other definitions for vowel that I've seen before include "A or e, say" , "Letter representing sound" , "Open sound in speech" , "A, E, I, O or U" , "aeio or u" .)