Foreign character sitting in restaurant (3)
I believe the answer is:
tau
'foreign character' is the definition.
(Greek letter)
'sitting in restaurant' is the wordplay.
'sitting in' indicates a hidden word.
'TAU' can be found hidden inside 'restaurant'.
(Other definitions for tau that I've seen before include "character from Athens" , "Homeric character" , "Marathon character" , "Greek character" , "Letter from Greece" .)