Remnant of brass tubing (4)
I believe the answer is:
stub
'remnant' is the definition.
(thesaurus)
'of brass tubing' is the wordplay.
'of' indicates the answer is hidden within the clue.
'STUB' is hidden in the letters of 'brass tubing'.
(Other definitions for stub that I've seen before include "End of cigarette or counterfoil of cheque" , "Short piece left over" , "Remnant, butt" , "Fag-end of cigerette or cheque counterfoil" , "Wee bit of pencil, eg," .)