Beginning of jarring in car manufacturer's inlets (6)
I believe the answer is:
fjords
'inlets' is the definition.
(fjord is a kind of inlet)
'beginning of jarring in car manufacturer's' is the wordplay.
'beginning of' indicates taking the first letters.
'in' indicates putting letters inside.
'car manufacturer's' becomes 'fords'.
The initial letter of 'jarring' is 'j'.
'j' going inside 'fords' is 'FJORDS'.
(Other definitions for fjords that I've seen before include "rocky inlets" , "Norwegian inlets of the sea" , "More than one Channel" , "Long narrow sea inlets between steep cliffs in Norway" , "Rock-bound inlets" .)