Soldiers going round at speed (4)
I believe the answer is:
rate
'speed' is the definition.
(rate is a kind of speed)
'soldiers going round at' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'going round' indicates putting letters inside.
're' going around 'at' is 'RATE'.
(Other definitions for rate that I've seen before include "Estimate value of - speed" , "The percentage of a loan charged as interest" , "Tear (anag.)" , "Relative speed (of progress)" , "The price paid" .)