Part of the anatomy of a girl of ten (6)
I believe the answer is:
thorax
'part' is the definition.
(thorax is a kind of part)
'anatomy of a girl of ten' is the wordplay.
'anatomy of a girl of' becomes 'thora' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ten' becomes 'X' (Roman numeral).
'thora'+'x'='THORAX'
'of the' is the link.
Can you help me to learn more?
(Other definitions for thorax that I've seen before include "Part of the body between the head and abdomen" , "Part of the human torso between neck and diaphragm" , "Middle section of insect's body" , "Middle section of an insect" , "Insect's upper body" .)