An explanation from engineers when working (6)
I believe the answer is:
reason
'an explanation' is the definition.
(reason is a kind of explanation)
'engineers when working' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'when' becomes 'as'.
'working' becomes 'on'.
're'+'as'+'on'='REASON'
'from' acts as a link.
(Other definitions for reason that I've seen before include "Employ logic" , "Sanity, the ability to think" , "Power of thought" , "Cause for sanity" , "Think in a logical way" .)