Express indecision on science of extraneous matter (4)
I believe the answer is:
scum
'express indecision on science of extraneous matter' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
'express indecision on science' is the wordplay.
'express indecision' becomes 'um' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'science' becomes 'sc'.
'um' after 'sc' is 'SCUM'.
Can you help me to learn more?
(Other definitions for scum that I've seen before include "Impurities on surface of liquid - or bad people" , "Impurities floating on water, or despicable people" , "Floating rubbish" , "Dirty foam" , "lowlife" .)