Being catty, feel upset about a bit of pain (6)
I believe the answer is:
feline
'being catty' is the definition.
(I've seen this in another clue)
'feel upset about a bit of pain' is the wordplay.
'upset' indicates anagramming the letters.
'about' is an insertion indicator.
'a bit of pain' becomes 'in' (I can't justify this - if you can you should believe this answer much more).
'feel' with letters rearranged gives 'fele'.
'fele' going around 'in' is 'FELINE'.
Can you help me to learn more?
(Other definitions for feline that I've seen before include "Like a cat" , "Relating to cats" , "Of or like cats" , "Pertaining to cats" , "Scots dish" .)