Boats, smooth and glossy, capsized (5)
I believe the answer is:
keels
'boats' is the definition.
(keel can mean a boat)
'smooth and glossy capsized' is the wordplay.
'smooth and glossy' becomes 'sleek' (I've seen this in another clue).
'capsized' says the letters should be written in reverse.
'sleek' back-to-front is 'KEELS'.
(Other definitions for keels that I've seen before include "bases of vessels" , "Parts of ships" .)