Respond to soldiers and behave (5)
I believe the answer is:
react
'respond' is the definition.
(I've seen this before)
'soldiers and behave' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'and' says to put letters next to each other.
'behave' becomes 'act' (acting is a kind of behaving).
're'+'act'='REACT'
'to' acts as a link.
(Other definitions for react that I've seen before include "Behave in response" , "Hit back" , "Take" , "What a chemical might do" , "provide an answer" .)