Mechanic has sister holding frame (7)
I believe the answer is:
chassis
'frame' is the definition.
(base frame of a vehicle)
'mechanic has sister holding' is the wordplay.
'holding' says the answer is hidden in the clue.
'CHASSIS' can be found hidden inside 'mechanic has sister'.
(Other definitions for chassis that I've seen before include "Vehicle's structure" , "Undercarriage" , "Vehicle structure" , "Mounting" , "Framework of motor vehicle" .)