Military action intended, we hear, to get part of the fortress (10)
I believe the answer is:
battlement
'part of the fortress' is the definition.
('battlement' is part of 'fortress')
'military action intended we hear' is the wordplay.
'military action' becomes 'battle' (battle is a kind of military action).
'intended' becomes 'meant' (to mean something is to intend it).
'we hear' indicates a 'sounds like' (homophone) clue.
'meant' is a homophone of 'ment'.
'battle'+'ment'='BATTLEMENT'
'to get' is the link.
(Other definitions for battlement that I've seen before include "parapet" , "Crenellation" , "fortification" , "Top of a fortified wall" .)