Bosom buddy heads out in large car (6)
I believe the answer is:
estate
'large car' is the definition.
(I've seen this before)
'bosom buddy heads out' is the wordplay.
'bosom' becomes 'est' (I can't explain this - if you can you should give a lot more credence to this answer).
'buddy' becomes 'mate' (I've seen this before).
'heads out' means to remove the first letter (I've seen 'heading out' mean this (the letter which heads the word is removed)).
'mate' with its initial letter taken off is 'ate'.
'est'+'ate'='ESTATE'
'in' is the link.
Can you help me to learn more?
(Other definitions for estate that I've seen before include "All of one's assets -- 5-door car" , "Family land" , "Property to be inherited" , "Group of suburban houses or industrial units" , "All money and property owned at time of death" .)