Associate harmoniously with board (3,2)
I believe the answer is:
'board' is the definition.
(I know that board can be written as get on)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for get on that I've seen before include "Make progress or mount up" , "Board - prosper" , "Make progress or advance in age" , "Hit it off" , "Make progress or climb aboard" .)