Soldiers see gun (3)
I believe the answer is:
rev
'gun' is the definition.
(as in gunning or revving up a car engine)
'soldiers see' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'see' becomes 'v' (short for 'vide', Latin meaning 'see elsewhere').
're'+'v'='REV'
(Other definitions for rev that I've seen before include "One turn of an engine, in short" , "In engine-room speed up" , "Run (engine) quickly" , "Gun (an engine)" , "Clergy title" .)