Signature diagram of vehicle? (9)
I believe the answer is:
autograph
'signature' is the definition.
(I know that autograph is a type of signature)
'diagram of vehicle?' is the wordplay.
I cannot really see how this works, but
'diagram' could be 'graph' (graphing is a kind of diagraming) and 'graph' is found within the answer.
'of' could be 'o' and 'o' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for autograph that I've seen before include "Sign" , "One's signature" , "(Celebrity's) signature" , "(Celebrity) signature" , "A signature given as souvenir" .)