Deck or dock (5)
I believe the answer is:
orlop
'deck' is the definition.
(orlop is a kind of deck)
'or dock' is the wordplay.
'dock' becomes 'lop'.
'or'+'lop'='ORLOP'
(Other definitions for orlop that I've seen before include "Lowest deck of a wooden sailing ship" , "deck at the lowest point" , "part of ship" , "Ship's lowest deck" , "hold covering" .)