Old man’s expression of thanks for Italian food (5)
I believe the answer is:
pasta
'italian food' is the definition.
(type of Italian food)
'old man's expression of thanks' is the wordplay.
'old man's' becomes 'pa's' (both can mean father).
'expression of thanks' becomes 'ta' (informal way of saying 'thank you').
'pas'+'ta'='PASTA'
'for' acts as a link.
(Other definitions for pasta that I've seen before include "The stuff of spaghetti, macaroni etc" , "E.g. fusilli" , "dish high in carbohydrates" , "Staple food of Italy" , "eg Macaroni" .)