One on board to put a thick black coat on (3)
I believe the answer is:
tar
'coat on' is the definition.
'tar' can be an answer for 'coat' (tarring is a kind of coating). I'm not certain of the 'on' bit.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for tar that I've seen before include "Informal sailor, the rat" , "Viscous bituminous substance" , "Viscous liquid; sailor" , "Liquid from wood, coal" , "Thick flammable liquid" .)