Short piece of brass tubing (4)
I believe the answer is:
stub
'short piece' is the definition.
(I've seen this in another clue)
'of brass tubing' is the wordplay.
'of' indicates a hidden word.
'STUB' is hidden within 'brass tubing'.
(Other definitions for stub that I've seen before include "Pencil or cigarette end" , "evidence of smoking, for example" , "Stump, of a cigarette say" , "Butt of pencil or cigarette" , "Tooth remnant" .)