Fall back and soldiers leap off, surrounding son (7)
I believe the answer is:
relapse
'fall back' is the definition.
('relapse' can be a synonym of 'fall back')
'soldiers leap off surrounding son' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'off' is an anagram indicator.
'surrounding' is an insertion indicator.
'son' becomes 's'.
'leap' with letters rearranged gives 'lape'.
'lape' going around 's' is 'lapse'.
're'+'lapse'='RELAPSE'
'and' is the link.
(Other definitions for relapse that I've seen before include "fall into sin" , "Fall ill again after rallying" , "Return to sickness" , "Fall back into sickness" , "Deterioration after period of improvement" .)