Bound to protect soldiers in ranks (6)
I believe the answer is:
tiered
'in ranks' is the definition.
(I've seen this before)
'bound to protect soldiers' is the wordplay.
'bound' becomes 'tied' (I've seen this before**).
'to protect' indicates putting letters inside.
'soldiers' becomes 're' (Royal Engineers).
'tied' enclosing 're' is 'TIERED'.
(Other definitions for tiered that I've seen before include "Built in layers" , "Set out in layers like wedding cake" , "Arranged in layers like big cake" , "Ranked" , "Arranged in rows" .)