Queen enters swimming pool in the lowest deck (5)
I believe the answer is:
orlop
'the lowest deck' is the definition.
(I've seen this before)
'queen enters swimming pool' is the wordplay.
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'enters' indicates putting letters inside.
'swimming' indicates anagramming the letters.
'pool' with letters rearranged gives 'olop'.
'r' placed within 'olop' is 'ORLOP'.
'in' acts as a link.
(Other definitions for orlop that I've seen before include "hold covering" , "part of ship" , "Ship's lowest deck" , "deck at the lowest point" , "Lowest deck of a wooden sailing ship" .)