One in vehicle looking in the mirror a lot? (4)
I believe the answer is:
vain
'looking in the mirror a lot?' is the definition.
(a vain person might look in the mirror a lot)
'one in vehicle' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'vehicle' becomes 'van' (van is a kind of vehicle).
'i' put inside 'van' is 'VAIN'.
(Other definitions for vain that I've seen before include "Conceited or unsuccessful" , "Useless; self-regarding" , "Stuck-up? It's hopeless" , "Conceited - futile" , "Egotistical" .)