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 they can define each other.
'many a vital part may' is the wordplay.
'many' becomes 'd'.
'part' suggests taking the first letters.
'may' becomes 'it' (I can't explain this - if you can you should believe this answer much more).
The initial letter of 'vital' is 'v'.
'd'+'a'+'v'+'it'='DAVIT'
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" .)