Man on board thrown vital rope, finally (6)
I believe the answer is:
castle
'man on board' is the definition.
(man can mean a chess piece)
'thrown vital rope finally' is the wordplay.
'thrown' becomes 'cast' (I've seen this in another clue).
'finally' indicates one should take the final letters.
The final letters of 'vital rope' are 'le'.
'cast'+'le'='CASTLE'
(Other definitions for castle that I've seen before include "Stately home in Yorkshire" , "Make chess move involving king and rook" , "Chess piece - stronghold" , "Another word for chess rook" , "Move king to safety - stronghold" .)