Fall back and soldiers leap off, surrounding son (7)
I believe the answer is:
Here is my best explanation:
'fall back' is the definition.
('relapse' can be a synonym of 'fall back')
'soldiers leap off surrounding son' is the subsidiary indication.
'soldiers' becomes 're' (Royal Engineers).
'off' is an anagram indicator.
'surrounding' indicates putting letters inside.
'son' becomes 's'.
'leap' with letters rearranged gives 'lape'.
'lape' going around 's' is 'lapse'.
'and' is the link.
(Other definitions for relapse that I've seen before include "Deterioration after period of improvement", "Fall back into sickness", "Repeals (anag.)", "Deterioration in health", "New downturn in health", "Slip back to former state", "Backsliding", "Regress, fall back", "Unexpected deterioration" and "Deteriorate in health".)