Old measure cut without bismuth (5)
I believe the answer is:
cubit
'old measure' is the definition.
(I've seen this before)
'cut without bismuth' is the wordplay.
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'bismuth' becomes 'Bi' (Bi is the chemical symbol for bismuth).
'cut' going around 'bi' is 'CUBIT'.
(Other definitions for cubit that I've seen before include "Old measure of length (from elbow to fingertips)" , "Elbow-to-finger measure" , "about eighteen inches" , "distance" , "Ancient linear measure" .)