Tall tales about gold rush's principal vehicles (7)
I believe the answer is:
lorries
'vehicles' is the definition.
(lorry is a kind of vehicle)
'tall tales about gold rush's principal' is the wordplay.
'tall tales' becomes 'lies' (I've seen this in another clue).
'about' is an insertion indicator.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'principal' indicates taking the first letters (the principal letter is the first letter).
The first letter of 'rushs' is 'r'.
'or'+'r'='orr'
'lies' placed around 'orr' is 'LORRIES'.
(Other definitions for lorries that I've seen before include "Large motor vehicles for carrying heavy goods" , "forms of transport" , "Trucks without sides" , "HGVs" , "Vehicles for transporting goods" .)