Set foot in gastro diner (4)
I believe the answer is:
trod
'set foot' is the definition.
(to tread somewhere is to set foot there)
'in gastro diner' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TROD' can be found hidden inside 'gastro diner'.
(Other definitions for trod that I've seen before include "start making wine" , "Crushed (grapes) by foot" , "Trampled" , "Walked in a specific way" , "Took steps" .)