It's useless putting one in a truck (4)
I believe the answer is:
vain
'it's useless' is the definition.
(thesaurus)
'one in a truck' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'a truck' becomes 'van' (van is a kind of truck).
'i' placed within 'van' is 'VAIN'.
'putting' is the link.
(Other definitions for vain that I've seen before include "Conceited or pointless" , "Conceited and ineffectual" , "being a big-head" , "Having an exaggerated sense of self-importance" , "Conceited, self-important" .)