As used by the navy and the horse artillery (3)
I believe the answer is:
sea
'as' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'by the navy and the horse artillery' is the wordplay.
'SEA' is hidden within the letters.
However, I'm uncertain how the hidden word is indicated.
'used' acts as a link.
Can you help me to learn more?
(Other definitions for sea that I've seen before include "Large expanse of water" , "Adriatic, for example" , "Part of ocean" , "The tide" , "that's easily reached from Dover?" .)