Sailor seeming to upset this supreme commander (13)
I believe the answer is:
generalissimo
'this supreme commander' is the definition.
(I've seen this before)
'sailor seeming to upset' is the wordplay.
'to upset' indicates anagramming the letters.
'sailor'+'seeming'='sailorseeming'
'sailorseeming' is an anagram of 'GENERALISSIMO'.
(Other definitions for generalissimo that I've seen before include "Political and military leader" , "Supreme commander (It.), title used by Franco" , "Top commander" , "Commander in chief" , "Commander-in-chief" .)