It's useless putting one in a truck i (4)
I believe the answer is:
vain
'it's useless' is the definition.
(I've seen this in another clue)
'one in a truck i' is the wordplay.
'one in' indicates putting letters inside.
'a truck' becomes 'van' (van is a kind of truck).
'van' enclosing 'i' is 'VAIN'.
'putting' is the link.
(Other definitions for vain that I've seen before include "Having an exaggerated sense of self-importance" , "Conceited or pointless" , "Stuck-up? It's hopeless" , "Useless; conceited" , "is full of himself" .)