Transport some soldiers in battle (7)
I believe the answer is:
freight
'transport' is the definition.
(freight is a kind of transport)
'some soldiers in battle' is the wordplay.
'some soldiers' becomes 're' (Royal Engineers).
'in' means one lot of letters goes inside another.
'battle' becomes 'fight' (fight is a kind of battle**).
're' inserted inside 'fight' is 'FREIGHT'.
(Other definitions for freight that I've seen before include "Goods in transit" , "Consignment" , "Goods carried by a large vehicle" , "Charge for carrying" , "Transport of bulk goods" .)