Spies fish served up in small bits (9)
I believe the answer is:
molecules
'spies' is the definition.
Although both the answer and definition are plural nouns, I don't understand how one could define the other.
'fish served up in small bits' is the wordplay.
'fish' becomes 'luce' (luce is a type of fish).
'served up' says the letters should be written backwards (in a down clue, going upwards).
'in' is an insertion indicator.
'small bits' becomes 'moles' (I can't explain this - if you can you should believe this answer much more).
'luce' backwards is 'ecul'.
'ecul' put within 'moles' is 'MOLECULES'.
Can you help me to learn more?
(Other definitions for molecules that I've seen before include "Small particles of matter" , "Mouse cell (Anag)" , "Simplest structural units of an element" .)