Doctor joins French friend on old Japanese craft (7)
I believe the answer is:
origami
'japanese craft' is the definition.
(I've seen this before)
'doctor joins french friend on old' is the wordplay.
'doctor joins' becomes 'rig' (I can't explain this - if you can you should believe this answer much more).
'french friend' becomes 'ami' ('friend' in French).
'on' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rig'+'ami'='rigami'
'rigami' after 'o' is 'ORIGAMI'.
(Other definitions for origami that I've seen before include "Art originally created in simple materials to design bird forms" , "Japanese art of folding or tearing paper" , "Oriental art of paper folding" , "Making artistic shapes out of paper" , "Paperwork, as it were" .)