Ultimately drives taxi to make a crust (4)
I believe the answer is:
scab
'a crust' is the definition.
(I've seen this in another clue)
'ultimately drives taxi' is the wordplay.
'ultimately' indicates one should take the final letters.
'taxi' becomes 'cab' (synonyms).
The last letter of 'drives' is 's'.
's'+'cab'='SCAB'
'to make' is the link.
(Other definitions for scab that I've seen before include "Crust-like surface over a wound" , "Dried crust on wound" , "complaint with scales" , "One regarded with contempt" , "one works during strike" .)