Aimlessly nod off in a vehicle (2,6)
I believe the answer is:
at random
'aimlessly' is the definition.
(thesaurus)
'nod off in a vehicle' is the wordplay.
'off' indicates an anagram.
'in' is an insertion indicator.
'vehicle' becomes 'tram' (tram is a kind of vehicle).
'nod' is an anagram of 'ndo'.
'a'+'tram'='atram'
'ndo' put inside 'atram' is 'AT RANDOM'.
(Other definitions for at random that I've seen before include "Without method or conscious decision" , "unplanned" , "willy-nilly" , "In no prearranged order" , "Indiscriminately" .)