Ultimately best butter in vehicle (4)
I believe the answer is:
tram
'vehicle' is the definition.
(tram is a kind of vehicle)
'ultimately best butter' is the wordplay.
'ultimately' indicates one should take the final letters.
'butter' becomes 'ram' (I've seen this in another clue).
The last letter of 'best' is 't'.
't'+'ram'='TRAM'
'in' acts as a link.
(Other definitions for tram that I've seen before include "multi-faceted yarn" , "Rail vehicle" , "Electrically driven transport" , "Old city transport, ram on rails through streets" , "The Luas, say" .)