Rotating platform finally built by high-class sailors on board (9)
I believe the answer is:
turntable
'rotating platform' is the definition.
(I've seen this before)
'finally built by high-class sailors on board' is the wordplay.
'finally' says to take the final letters.
'by' says to put letters next to each other.
'high class' becomes 'U' (as in U and non-U speech).
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'on board' becomes 'table' (both can mean food. I am not sure about the 'on' bit.).
The last letter of 'built' is 't'.
't'+'u'+'rn'+'table'='TURNTABLE'
(Other definitions for turntable that I've seen before include "part of record player" , "Rotating platform" , "it's spinning on the deck" , "Old record-holder" , "LP support" .)