He oils the engine and changes gears with hesitation (7)
I believe the answer is:
greaser
'he oils the engine' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'changes gears with hesitation' is the wordplay.
'changes' is an anagram indicator.
'with' is a charade indicator (letters next to each other).
'hesitation' becomes 'er' (sound made while hesitating - um, er).
'gears' with letters rearranged gives 'greas'.
'greas'+'er'='GREASER'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for greaser that I've seen before include "Biker" , "Mechanic - sycophant" , "Young motorcyclist" , "(Unskilled) mechanic" , "Ship's engineer; motor mechanic" .)