Using science, they designed tool (6)
I believe the answer is:
scythe
'tool' is the definition.
(scythe is a kind of tool)
'using science they designed' is the wordplay.
I cannot quite understand how this works, but
'science' could be 'sc' and 'sc' is found in the answer.
an anagram of 'they' is 'ythe' which is within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for scythe that I've seen before include "Grass-cutter" , "Manual hay-cutter wielded with both hands" , "farm hands once had to work with" , "Manual tool of reaper" , "Agricultural tool" .)