Not the first man to go on board (5)
I believe the answer is:
black
'board' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'not the first man to go on' is the wordplay.
'not the first' means to remove the first letter.
'man' becomes 'Al' (man's name).
'to go' becomes 'back' (backing is a kind of going).
'on' is an insertion indicator (as in clothing 'on' a person).
'al' with its initial letter taken away is 'l'.
'l' going inside 'back' is 'BLACK'.
Can you help me to learn more?
(Other definitions for black that I've seen before include "(Of coffee) without milk" , "Extremely dark" , "Colour of soot" , "bar" , "Can precede art, box, eye" .)