Quantity of wheat, ultimately, in old Italian bread (5)
I believe the answer is:
litre
'quantity' is the definition.
(litre is a kind of quantity)
'wheat ultimately in old italian bread' is the wordplay.
'ultimately' indicates one should take the final letters.
'in' is an insertion indicator.
'old italian bread' becomes 'lire' (I've seen this in another clue).
The final letter of 'wheat' is 't'.
't' put within 'lire' is 'LITRE'.
'of' acts as a link.
(Other definitions for litre that I've seen before include "1.7598 UK pints" , "non-imperial measure" , "Fluid measure" , "One thousand cubic centimetres" , "Nearly two pints" .)