Said what diner did, a curvy figure (5)
I believe the answer is:
'a curvy figure' is the definition.
(visually curvy number)
'said what diner did' is the wordplay.
'said' indicates a 'sounds like' (homophone) clue.
'what diner did' becomes 'ate' (a diner eats food).
'ate' sounds like 'EIGHT'.
(Other definitions for eight that I've seen before include "Perfect cube" , "4x2" , "Pints in gallon" , "Number for rowers" , "Number in a Boat Race crew" .)