Musician’s track mostly introduces a new one (7)
I believe the answer is:
pianist
'musician's' is the definition.
(pianist is a kind of musician)
'track mostly introduces a new one' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for pianist that I've seen before include "Person at keyboard" , "Ashkenazy or Lang Lang" , "Instrumentalist" , "Ivories tinkler" , "Key player" .)