Man on board’s obscene material spoken of (4)
I believe the answer is:
pawn
This clue might be a double definition.
'man on board's' is the first definition.
(chess)
'obscene material spoken of' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Other definitions for pawn that I've seen before include "Deposit as pledge" , "One getting used" , "A dupe; hock" , "Person one used" , "Lowest ranking chessman" .)