Particle to be inserted in the morning (4)
I believe the answer is:
atom
'particle' is the definition.
(synonyms)
'to be inserted in the morning' is the wordplay.
'be inserted' means one lot of letters goes inside another.
'in the morning' becomes 'am' (AM on a clock).
'to' going inside 'am' is 'ATOM'.
(Other definitions for atom that I've seen before include "Very small bit" , "A tiny particle but a big bomb" , "A really tiny piece of something" , "A wee bit" , "A mighty particle, can be explosive" .)