Small amount to put in before noon (4)
I believe the answer is:
atom
'small amount' is the definition.
(I've seen this in another clue)
'to put in before noon' is the wordplay.
'put in' indicates putting letters inside.
'before noon' becomes 'am' (as in AM on a clock).
'to' inserted within 'am' is 'ATOM'.
(Other definitions for atom that I've seen before include "A small but mighty particle" , "Particle; molecule" , "Nucleus plus electrons" , "Nucleus surrounded" , "Smallest particle in an element able to take part in a chemical reaction" .)