Sea creature needs to come out head last (7)
I believe the answer is:
manatee
'sea creature' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for manatee that I've seen before include "'Amen, tea for the aquatic mammal (7)'" , "water animal" , "To find sea cow" , "Large aquatic mammal -- sea cow" , "big creature" .)