Piano track without least jazzy old instrument (8)
I believe the answer is:
psaltery
'old instrument' is the definition.
(ancient string instrument)
'piano track without least jazzy' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'track' becomes 'ry' (abbreviation for railway).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'jazzy' indicates an anagram.
'p'+'ry'='pry'
'least' with letters rearranged gives 'salte'.
'pry' going around 'salte' is 'PSALTERY'.
(Other definitions for psaltery that I've seen before include "Ancient plucked instrument" , "Old instrument like a dulcimer" , "plucky thing" , "string instrument of yore" , "Plucked instrument; play rest (anag.)" .)