Otherwise cut lower deck (5)
I believe the answer is:
orlop
'lower deck' is the definition.
(I've seen this in another clue)
'otherwise cut' is the wordplay.
'otherwise' becomes 'or'.
'cut' becomes 'lop' (lopping is a kind of cutting).
'or'+'lop'='ORLOP'
(Other definitions for orlop that I've seen before include "hold covering" , "Lowest deck of a wooden sailing ship" , "Ship's lowest deck" , "part of ship" , "deck at the lowest point" .)