Momentarily, you may have a hand in it (6)
I believe the answer is:
sleeve
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sleeve that I've seen before include "Record-holder" , "Tube" , "Part of a shirt - old record holder" , "It covers the arm" , "Some wear their hearts on it" .)