Transport mechanism's principal emblem (5)
I believe the answer is:
totem
'emblem' is the definition.
(totem is a kind of emblem)
'transport mechanism's principal' is the wordplay.
'transport' becomes 'tote' (toting is a kind of transporting**).
'principal' suggests taking the first letters.
The first letter of 'mechanism' is 'm'.
'tote'+'m'='TOTEM'
(Other definitions for totem that I've seen before include "Thing of veneration (on a pole?)" , "Spiritually significant item" , "Emblem, maybe on pole" , "Emblem believed to have spiritual significance" , "Animal as emblem" .)