It's useful to sailors and the horse artillery (3)
I believe the answer is:
sea
'it's useful to sailors and' is the definition.
I can't tell whether this definition defines the answer.
'horse artillery' is the wordplay.
'SEA' is hidden within the letters.
However, I'm unsure how the hidden word is indicated.
'the' is the link.
Can you help me to learn more?
(Other definitions for sea that I've seen before include "The main" , "Ocean area" , "Division of an ocean" , "Lesser ocean" , "Part of ocean" .)