A giant box containing soldiers (5)
I believe the answer is:
titan
'a giant' is the definition.
(I know that giant can be written as titan)
'box containing soldiers' is the wordplay.
'box' becomes 'tin' (I've seen this in other clues).
'containing' means one lot of letters goes inside another.
'soldiers' becomes 'TA' (abbreviation for Territorial Army).
'tin' going around 'ta' is 'TITAN'.
(Other definitions for titan that I've seen before include "enormous thing" , "colossal one" , "An old Greek god - Atlas was one" , "Huge character" , "Atlas, possibly" .)