Army units on top defended by engineers (8)
I believe the answer is:
platoons
'army units' is the definition.
(platoon is a kind of army unit)
'on top defended by engineers' is the wordplay.
'on top' becomes 'too' (on top can mean in addition or as well).
'defended by' is an insertion indicator.
'engineers' becomes 'plans' (engineer can mean to plan or manipulate).
'too' inserted into 'plans' is 'PLATOONS'.
(Other definitions for platoons that I've seen before include "Army units" , "small bodies of soldiers" , "Military units" .)