As a rule Sun has nice characters upset (7)
I believe the answer is:
arsenic
'upset' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'as a rule sun has nice characters' is the wordplay.
'as a rule' becomes 'ar' (this might be a standard abbreviation which I don't know about).
'sun' becomes 's'.
'has' says to put letters next to each other.
'characters' indicates anagramming the letters.
'nice' with letters rearranged gives 'enic'.
'ar'+'s'+'enic'='ARSENIC'
Can you help me to learn more?
(Other definitions for arsenic that I've seen before include "Poisons in scare" , ", it goes with old lace" , "Famous stage comedy ''. . . . . . . and Old Lace''" , "Ratsbane" , "-- and Old Lace (play)" .)