Design detail, briefly (4)
I believe the answer is:
spec
'design detail briefly' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for spec that I've seen before include "estimation?" , "Detailed design document (abbr.)" , "Estimate" , "on which chance is taken" , "On - means without a plan but with hope of success" .)