Bert's upset about it being cruel (6)
I believe the answer is:
bitter
'cruel' is the definition.
(thesaurus)
'bert's upset about it' is the wordplay.
'upset' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'bert' is an anagram of 'bter'.
'bter' enclosing 'it' is 'BITTER'.
'being' is the link.
(Other definitions for bitter that I've seen before include "Having a sharp, acid taste" , "Sarcastic (words); very cold" , "Sharp-tasting or resentful" , "Causing unhappiness" , "Act nervously" .)