You can't miss it on board at sea before year's end (4,4)
I believe the answer is:
barn door
'you can't miss it' is the definition.
(I've seen this before)
'on board at sea before year's end' is the wordplay.
'at sea' is an anagram indicator.
'before' means one lot of letters go next to another.
'end' suggests the final letters.
The last letter of 'year' is 'r'.
'on'+'board'='onboard'
'onboard' anagrammed gives 'barndoo'.
'barndoo'+'r'='BARN-DOOR'
(Other definitions for barn door that I've seen before include "Unmissable target" , "Easy target" , "you can't miss it" , "No point in locking it after the horse has bolted" , "What a bad shot can't hit" .)