New onboard motor’s last thing you should hit! (4,4)
I believe the answer is:
barn door
'thing you should hit' is the definition.
(as in phrases about hitting a barn door)
'new onboard motor's last' is the wordplay.
'new' indicates anagramming the letters.
'last' says to take the final letters.
The last letter of 'motor' is 'r'.
'onboard' with letters rearranged gives 'barndoo'.
'barndoo'+'r'='BARN-DOOR'
(Other definitions for barn door that I've seen before include "Unmissable target" , "No point in locking it after the horse has bolted" , "Easy target" , "you can't miss it" , "What a bad shot can't hit" .)