On board, it's not a friendly thing to break (6)
I believe the answer is:
notice
'on board' is the definition.
'notice' can be an answer for 'board' (notice board is a kind of board). I am unsure of the 'on' bit.
'it's not a friendly thing to break' is the wordplay.
I cannot quite see how this works, but
'not' could be 'no' (I've seen this in other clues) and 'no' is located in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'ice' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for notice that I've seen before include "Announcement (of the sack?)" , "Don't miss - review" , "Formal advice - to quit?" , "Observe - about to resign?" , "Advance warning given of resignation" .)