Replace soldiers, as it happens, touring East (7)
I believe the answer is:
relieve
'replace' is the definition.
(thesaurus)
'soldiers as it happens touring east' is the wordplay.
'soldiers' becomes 'RE' (Royal Engineers).
'as it happens' becomes 'live' (as in live television).
'touring' means one lot of letters goes inside another (touring can mean going round).
'east' becomes 'E' (synonyms).
'live' enclosing 'e' is 'lieve'.
're'+'lieve'='RELIEVE'
(Other definitions for relieve that I've seen before include "Alleviate pain or stress" , "help" , "Take a job over from - bring aid" , "Break the sameness of" , "cure" .)