At sea, repeat securing unknown yachtsman’s harness (7)
I believe the answer is:
trapeze
'yachtsman's harness' is the definition.
(type of harness used in sailing)
'at sea repeat securing unknown' is the wordplay.
'at sea' indicates anagramming the letters.
'securing' is an insertion indicator.
'unknown' becomes 'z' (algebra).
'repeat' anagrammed gives 'trapee'.
'trapee' placed around 'z' is 'TRAPEZE'.
(Other definitions for trapeze that I've seen before include "swingers' bar" , "Flying bar for acrobatics" , "Six down's swing" , "Swinging acrobat's bar" , "Acrobatic apparatus" .)