In shop a starter is Italian staple (5)
I believe the answer is:
pasta
'italian staple' is the definition.
(I've seen this in another clue)
'in shop a starter' is the wordplay.
'in' indicates the answer is hidden within the clue.
'PASTA' is hidden within 'shop a starter'.
'is' is the link.
(Other definitions for pasta that I've seen before include "Staple food of Italian origin" , "Penne, rigatoni etc" , "Eg, lasagne" , "Italian foodstuff" , "Flour and water foodstuff" .)