Associate harmoniously with board (3,2)
I believe the answer is:
get on
'board' is the definition.
(synonyms)
I can't explain the remainder 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" .)