Soldiers can use empty train (7)
I believe the answer is:
retinue
'train' is the definition.
(people following someone in a procession)
'soldiers can use empty' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'can' becomes 'tin' (both are metal containers).
'empty' says to hollow out the word (remove centre letters).
'use' with its middle removed is 'ue'.
're'+'tin'+'ue'='RETINUE'
(Other definitions for retinue that I've seen before include "Entourage, followers" , "Group around VIP" , "Group of assistants" , "Cortege" , "Group attending a person" .)