If nothing else, she's hard to upset (5)
I believe the answer is:
rhoda
'upset' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'if nothing else she's hard' is the wordplay.
I cannot really understand how this works, but
'nothing' could be 'o' (O resembles 0) and 'o' is located in the answer.
an anagram of 'hard' is 'rhda' which is located in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for rhoda that I've seen before include "Fem. name; hoard (anag.)" , "Girl" , "Woman" , "Girl's name; hoard (anag.)" .)