In the army, it means “attention!” (3)
I believe the answer is:
ear
'attention' is the definition.
(ear is a kind of attention)
'in the army it' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EAR' is hidden in the letters of 'the army it'.
'means' is the link.
(Other definitions for ear that I've seen before include "Seed-bearing part of corn" , "Heed" , "Attention; seed head" , "Shell-like; corn" , "one hears" .)