Part of kit containing posh ingredient for making pasta (5)
I believe the answer is:
durum
'ingredient for making pasta' is the definition.
(durum wheat)
'part of kit containing posh' is the wordplay.
'part of kit' becomes 'drum' (part of a drum kit).
'containing' is an insertion indicator.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'drum' going around 'u' is 'DURUM'.
(Other definitions for durum that I've seen before include "Type of flour used for pasta" , "Wheat for pasta flour" , "Hard type of wheat" , "Kind of wheat used to make pasta" , "Wheat variety" .)