Falls back, seeing mistakes by engineers (8)
I believe the answer is:
relapses
'falls back' is the definition.
('relapse' can be a synonym of 'fall back')
'mistakes by engineers' is the wordplay.
'mistakes' becomes 'lapses' (lapse is a kind of mistake**).
'by' means one lot of letters go next to another.
'engineers' becomes 're' (Royal Engineers).
'lapses' put after 're' is 'RELAPSES'.
'seeing' acts as a link.
(Other definitions for relapses that I've seen before include "Deteriorates like pleasers" , "Regresses" , "Falls back to former condition, perhaps of illness" , "becomes ill again" , "Backslides like pleasers" .)