Prompt plonker to embark (5,5)
I believe the answer is:
idiot board
'prompt' is the definition.
(I've seen this before)
'plonker to embark' is the wordplay.
'plonker' becomes 'idiot' (both can mean a stupid person).
'to embark' becomes 'board' (I've seen this in another clue).
'idiot'+'board'='IDIOT BOARD'
(Other definitions for idiot board that I've seen before include "Autocue" , "prompt" .)