Craft gathering round a major vessel (5)
I believe the answer is:
aorta
'major vessel' is the definition.
(I've seen this in another clue)
'craft gathering round a' is the wordplay.
'craft' becomes 'art' (both can mean cunning or trickery).
'gathering' means one lot of letters goes inside another.
'round' becomes 'o' (round can mean a round object).
'art' placed around 'o' is 'aort'.
'aort'+'a'='AORTA'
(Other definitions for aorta that I've seen before include "sort of vessel" , "claret supplier" , "blood flows from it" , "Main arterial vessel, carrying blood to the heart" , "vital transporter" .)