Engineer unavailable to fill ditch (6)
I believe the answer is:
boffin
'engineer' is the definition.
(boffin can informally mean a scientist)
'unavailable to fill ditch' is the wordplay.
'unavailable' becomes 'off'.
'to fill' is an insertion indicator.
'ditch' becomes 'bin' (both can mean to reject or throw away).
'off' inserted within 'bin' is 'BOFFIN'.
(Other definitions for boffin that I've seen before include "(Government) scientist" , "Research scientist" , "Technical researcher" , "Scientist (in the back room?)" , "Brainbox" .)