Big vessel in sea or tank (5)
I believe the answer is:
aorta
'big vessel' is the definition.
(I've seen this in another clue)
'in sea or tank' is the wordplay.
'in' indicates a hidden word.
'AORTA' is hidden in the letters of 'sea or tank'.
(Other definitions for aorta that I've seen before include "vital transporter" , "This carries blood from the heart" , "One leads from heart" , "Main artery from heart" , "Trunk artery from the heart" .)