Fall back, mistake made by soldiers (7)
I believe the answer is:
relapse
'fall back' is the definition.
(synonyms)
'mistake made by soldiers' is the wordplay.
'mistake' becomes 'lapse' (lapse is a kind of mistake).
'made by' means one lot of letters go next to another (letters written or made by another set of letters).
'soldiers' becomes 'RE' (Royal Engineers).
'lapse' put after 're' is 'RELAPSE'.
(Other definitions for relapse that I've seen before include "A return to illness" , "Pleaser may slip to earlier state" , "setback in health terms" , "Degenerate like pleaser" , "Fall back (into ill health)" .)