One egg you should sit on (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'egg you should sit on' is the wordplay.
I cannot really see how this works, but
'egg' could be 'nit' (a nit is an insect egg) and 'nit' is found in the answer.
'you' could be 'u' (commonly used in online communication) and 'u' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for unit that I've seen before include "Single undivided whole" , "- - trust" , "Self-contained part" , "Module" , "Element or part" .)