Support US soldier in ebbing sea (5)
I believe the answer is:
aegis
'support' is the definition.
(I know that aegis is a type of endorsement)
'us soldier in ebbing sea' is the wordplay.
'us soldier' becomes 'GI' (informal term for a US soldier).
'in' means one lot of letters goes inside another.
'ebbing' is a reversal indicator (to ebb can mean to go backwards).
'sea' back-to-front is 'aes'.
'gi' going into 'aes' is 'AEGIS'.
(Other definitions for aegis that I've seen before include "Protection - patronage" , "Protection, backing" , "Advocacy" , "Custody" , "Backing or support" .)