One form of pasta if potato, say, is short (4)
I believe the answer is:
tube
'one form of pasta' is the definition.
(eg penne, tube-shaped pasta)
'potato say is short' is the wordplay.
'potato say' becomes 'tuber' (type of tuber).
'is short' means to remove the last letter.
'tuber' with its last letter taken away is 'TUBE'.
'if' acts as a link.
(Other definitions for tube that I've seen before include "Cylindrical hollow conduit" , "Pack of toothpaste; type of train" , "Hollow cylindrical object" , "London Underground or long cylinder" , "Underground transport" .)