Stop on board (5,2)
'stop' is the definition.
(I've seen this before)
'on board' is the wordplay.
I cannot really understand how this works, but
'on' could be 'to' and 'to' is found within the answer.
The remaining letters 'heave' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for heave to that I've seen before include "stop at sea" , "Of ship, stop without anchoring or mooring" , "come to a stop for navy" , "(Of a sailing vessel) stop moving" .)
'stop on board' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for avast that I've seen before include "Stop, nautically speaking" , ""stop" signal" , "Stop! (naut.)" , "Stop sailing" , "shout stopping the ship" .)