Blueprint bit (4)
I believe the answer is:
spec
'blueprint bit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spec that I've seen before include "commercial venture" , "Detailed description of design criteria (abbr)" , "Detailed design (colloq.)" , "On - means on the chance of achieving something" , "Required standard of workmanship" .)