Mere nit-picking, it gets Calvin no end upset! (5)
I believe the answer is:
cavil
'mere nit-picking it' is the definition.
I can't judge whether this definition defines the answer.
'calvin no end upset' is the wordplay.
'no end' means to remove the last letter.
'upset' indicates anagramming the letters.
'calvin' with its final letter removed is 'calvi'.
'calvi' anagrammed gives 'CAVIL'.
'gets' is the link.
Can you help me to learn more?
(Other definitions for cavil that I've seen before include "Complain" , "Minor complaint" , "Raise petty objections" , "Make trivial objection" , "Carp" .)