Many a vital part may project overboard (5)
I believe the answer is:
davit
'project overboard' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'many a vital part' is the wordplay.
'many' becomes 'D' (Roman numeral for 500).
'a vital part' becomes 'avit' (I can't justify this - if you can you should give a lot more credence to this answer).
'd'+'avit'='DAVIT'
'may' is the link.
Can you help me to learn more?
(Other definitions for davit that I've seen before include "Lifeboat crane" , "Crane-like device for suspending or lowering ship's boats" , "device for lowering boat" , "nautical equipment" .)