Being catty, feel upset about a certain measure (6)
I believe the answer is:
feline
'being catty' is the definition.
(I've seen this in another clue)
'feel upset about a certain measure' is the wordplay.
'upset' is an anagram indicator.
'about' means one lot of letters goes inside another.
'a certain measure' becomes 'in' (in is a kind of measure. I am not sure about the 'certain' bit.).
'feel' anagrammed 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 "Relating to cats" , "Like a leopard, say" , "suitable for Skimbleshanks?" , "(Relating to a) cat" , "Like a cat" .)