Form of pasta (5)
(Other definitions for penne that I've seen before include "2 [PASTA]" , "Pasta in short tubes" , "Tube pasta" , "Tubular pasta" , "Wide tube-shaped pasta" .)
'form of pasta' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for spaghetti that I've seen before include "Pasta in the form of long strings" , "Italian food" , "stringy food" , "Variety of pasta" , "something resembling tangled wires" .)