Sea horse not holding out! (6)

I believe the answer is:

marine

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'sea' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for marine that I've seen before include "Airmen (anag.)" , "Relating to the sea" , "Member of the forces" , "'Nautical, maritime (6)'" , "seaman" .)

I've seen this clue in the Evening Standard.

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