Keep tight hold of part of the car's mechanism (6)
I believe the answer is:
clutch
This clue is a quintuple definition!!
'keep tight' is the first definition.
'clutch' can be an answer for 'keep' (thesaurus). I'm unsure of the 'tight' bit.
'hold' is the second definition.
(synonyms)
'of' acts as a link.
'part' is the third definition.
(thesaurus)
'of' is a link.
'the car's' is the fourth definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'mechanism' is the fifth definition.
(clutch is a kind of mechanism)
Can you help me to learn more?
(Other definitions for clutch that I've seen before include "Grab - set of eggs" , "Grasp - gear changing device" , "Hold tight to the eggs" , "Grip part of the car-engine" , "Grip tightly for car engine part" .)