Popular commercial vehicle carries one to no avail (2,4)
I believe the answer is:
in vain
'to no avail' is the definition.
('in vain' can be a synonym of 'to no avail')
'popular commercial vehicle carries one' is the wordplay.
'popular' becomes 'in'.
'commercial vehicle' becomes 'van' (I've seen this before).
'carries' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'van' enclosing 'i' is 'vain'.
'in'+'vain'='IN VAIN'
(Other definitions for in vain that I've seen before include "Without achieving anything" , "Unsuccessfully" , "Without success or result" , "Uselessly, without result" , "'To no avail (2,4)'" .)