Said to be the part of the wheel at the back of a wagon (6)
I believe the answer is:
spoken
'said' is the definition.
(I've seen this before)
'the part of the wheel at the back of a wagon' is the wordplay.
'the part of the wheel' becomes 'spoke' ('spoke' is part of 'wheel').
'at the back of' suggests the final letters (I've seen 'at the back' mean this).
The last letter of 'wagon' is 'n'.
'spoke'+'n'='SPOKEN'
'to be' acts as a link.
(Other definitions for spoken that I've seen before include "Said" , "Oral" , "Uttered through the medium of speech" , "Voiced" , "Verbalised" .)