One fragment, one artery (5)
I believe the answer is:
aorta
'artery' is the definition.
(aorta is a kind of artery)
'one fragment one' is the wordplay.
'one' becomes 'a'.
'fragment' becomes 'ort' (I've seen this before).
'one' becomes 'a'.
'a'+'ort'+'a'='AORTA'
(Other definitions for aorta that I've seen before include "Trunk artery from the heart" , "Large artery of the heart" , "Trunk route" , "sort of vessel" , "Major artery of the body" .)