Tonic the setter found in a truck (4-2-2)
I believe the answer is:
pick-me-up
'tonic' is the definition.
(I've seen this before)
'setter found in a truck' is the wordplay.
'setter' becomes 'me' (the setter of the clue).
'found in' indicates putting letters inside.
'a truck' becomes 'pickup' (pick-up is a kind of truck).
'me' inserted inside 'pickup' is 'PICK-ME-UP'.
'the' is the link.
(Other definitions for pick-me-up that I've seen before include "Stimulative drink" , "I could do with a lift" , "Stimulant" , "Medicinal tonic" , "A restorative tonic drink" .)