Foot in this person's vehicle (6)
I believe the answer is:
iambus
'foot' is the definition.
(iambus is a kind of foot)
'this person's vehicle' is the wordplay.
'this person's' becomes 'iam' ('this person' being the crossword setter).
'vehicle' becomes 'bus' (I've seen this before).
'iam'+'bus'='IAMBUS'
'in' acts as a link.
(Other definitions for iambus that I've seen before include "Metrical foot" , "Metrical unit of two syllables" , "poetic form" , "Type of metrical unit in a poem" .)