Main part of the horse artillery (3)
I believe the answer is:
sea
'main' is the definition.
(the main is an old name for the sea)
'part of the horse artillery' is the wordplay.
'part of' says the answer is hidden in the clue.
'SEA' is hidden in the letters of 'the horse artillery'.
(Other definitions for sea that I've seen before include "The Med, e.g." , "Ocean area" , "The tide" , "E.g. Yellow, Red area" , "The deep" .)