Queen boards large sailing ship (4)
I believe the answer is:
brig
'sailing ship' is the definition.
(brig is a kind of sailing ship)
'queen boards large' is the wordplay.
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'boards' means one lot of letters goes inside another.
'large' becomes 'big' ('big' can be a synonym of 'large'**).
'r' placed into 'big' is 'BRIG'.
(Other definitions for brig that I've seen before include "staff officer briefly used" , "'Two-masted,square-rigged ship (4)'" , "Sailing vessel or lock-up on board" , "Two-masted square-rigger" , "senior officer briefly used" .)