Minute particle to be seen in the morning (4)
I believe the answer is:
atom
'minute particle' is the definition.
(I've seen this in another clue)
'to be seen in the morning' is the wordplay.
'be seen in' means one lot of letters goes inside another.
'the morning' becomes 'am' (as in 12:00 AM).
'to' going within 'am' is 'ATOM'.
(Other definitions for atom that I've seen before include "Nucleus plus electrons" , "A small but mighty particle" , "little amount" , "Not much substance" , "Tiny particle, great bomb" .)