Surfeit starts to show at one over eight, say (7)
I believe the answer is:
satiate
'surfeit' is the definition.
(I've seen this in another clue)
'starts to show at one over eight say' is the wordplay.
'starts to' suggests taking the first letters.
'one' becomes 'i' (Roman numeral).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'say' shows a homophone (sound like).
The first letter of 'show' is 's'.
'eight' sounds like 'ate'.
's'+'at'+'i'+'ate'='SATIATE'
(Other definitions for satiate that I've seen before include "content" , "Overstuff" , "gladden a gourmand" , "Satisfy fully, cloy with excess" , "Completely satisfy" .)