Conclude sappers like working (6)
I believe the answer is:
reason
'conclude' is the definition.
('reason' can be a synonym of 'conclude')
'sappers like working' is the wordplay.
'sappers' becomes 're' (Royal Engineers).
'like' becomes 'as'.
'working' becomes 'on'.
're'+'as'+'on'='REASON'
(Other definitions for reason that I've seen before include "Explanation; logic" , "Explanation for some event" , "Explanation or justification" , "Logic and rationality" , "Capacity for rational thought" .)