Unwrap wintry onesie and see if it fits (3,2)
I believe the answer is:
try on
'it fits' is the definition.
The definition suggests a singular noun which matches the answer.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for try on that I've seen before include "Test a garment's size and look" , "attempt to deceive" , "attempted deception" , "attempt to sneak an advantage?" , "Test a garment for fit" .)