A sunken ship, some crew reckon (5)
I believe the answer is:
wreck
'a sunken ship' is the definition.
(I've seen this before)
'some crew reckon' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'WRECK' is hidden within 'crew reckon'.
(Other definitions for wreck that I've seen before include "Destroy; sunken vessel" , "Completely spoil" , "Devastate" , "Ruin; shatter" , "A destroyed ship" .)