Starts to grin, upsetting young kid (3)
I believe the answer is:
guy
'kid' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'starts to grin upsetting young' is the wordplay.
'starts to' suggests taking the first letters.
The first letters of 'grin upsetting young' is 'GUY'.
Can you help me to learn more?
(Other definitions for guy that I've seen before include "One burned on Nov. fifth" , "Bloke" , "Make fun of, tent rope" , "Cable, wire etc used to brace something" , "Ridicule like Fawkes" .)