Vessel seen in Zambia or Tanzania (5)
I believe the answer is:
aorta
'vessel' is the definition.
(blood vessel)
'in zambia or tanzania' is the wordplay.
'in' indicates the answer is hidden within the clue.
'AORTA' is hidden within 'zambia or tanzania'.
'seen' acts as a link.
(Other definitions for aorta that I've seen before include "it's drawing blood!" , "claret supplier" , "Large artery of the heart" , "Main route from the heart" , "vital transporter" .)