One’s hiding the onset of fresh uncertainties (3)
I believe the answer is:
ifs
'uncertainties' is the definition.
(an if is an uncertainty or something in doubt)
'one's hiding the onset of fresh' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'hiding' means one lot of letters goes inside another.
'the onset of' suggests taking the first letters.
The first letter of 'fresh' is 'f'.
'is' enclosing 'f' is 'IFS'.
(Other definitions for ifs that I've seen before include "No ...,ands or buts, no argument allowed" , "Conditional conjunctions" , "If . . . and ands were pots and pans" , "Conditions or suppositions, may join with buts" , "Suppositions, may be accompanied by buts" .)