Mistake by sappers — fall back (7)
I believe the answer is:
relapse
'fall back' is the definition.
(synonyms)
'mistake by sappers' is the wordplay.
'mistake' becomes 'lapse' (lapse is a kind of mistake).
'by' says to put letters next to each other.
'sappers' becomes 'RE' (Royal Engineers).
'lapse' put after 're' is 'RELAPSE'.
(Other definitions for relapse that I've seen before include "falling back" , "Decline" , "causing setback to recovery" , "Fall back into former condition, being pleaser" , "setback in health terms" .)