Musicians keep shaking transport that's jumped on (9)
I believe the answer is:
bandwagon
'transport that's jumped on' is the definition.
(as in jumping on a bandwagon)
'musicians keep shaking' is the wordplay.
'musicians' becomes 'band' (group of musicians).
'keep shaking' becomes 'wag on' (wag on could mean to continue wagging or shaking).
'band'+'wagon'='BANDWAGON'
(Other definitions for bandwagon that I've seen before include "something fashionable all of a sudden" , "Vehicle (jumped on when suddenly popular?)" , "everybody wants to be on it" , "vehicle for musicians" , "Suddenly fashionable cause or activity" .)