Little things to put in a manuscript (5)
I believe the answer is:
atoms
'little things' is the definition.
(I've seen this before)
'to put in a manuscript' is the wordplay.
'put in' is an insertion indicator.
'manuscript' becomes 'ms' (abbreviation for manuscript).
'a'+'ms'='ams'
'to' inserted within 'ams' is 'ATOMS'.
(Other definitions for atoms that I've seen before include "Little things" , "small fragments" , "Dots" , "Tiny particles" , "Small particles" .)