Mother in car with heavyweight robot (9)
I believe the answer is:
automaton
'robot' is the definition.
(synonyms)
'mother in car with heavyweight' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'in' is an insertion indicator.
'car' becomes 'auto' (automobile).
'with' means one lot of letters go next to another.
'heavyweight' becomes 'ton' (a ton weighs a lot).
'auto'+'ton'='autoton'
'ma' put inside 'autoton' is 'AUTOMATON'.
(Other definitions for automaton that I've seen before include "Amount to a (anag) - robot" , "Mechanical man" , "being worked invariably" , "Mechanical figure" , "machine" .)