Oily part of a tractor (3)
I believe the answer is:
fat
'oily' is the definition.
(similar in meaning)
'part of a tractor' is the wordplay.
'part' says the answer is hidden in the clue.
'FAT' is hidden in the letters of 'of a tractor'.
(Other definitions for fat that I've seen before include "eg Lard" , "Portly" , "As Bunter" , "Overweight, stout" , "Naturally oily substance" .)