Crossword Genius

If Fred's upset, is not at all agreeable! (7)

Ross

I believe the answer is:

differs

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'not at all agreeable' is the definition.
I can't tell whether this defines the answer.

'if fred's upset' is the wordplay.
'upset' indicates anagramming the letters.
'if'+'freds'='iffreds'
'iffreds' anagrammed gives 'DIFFERS'.

'is' acts as a link.

Can you help me to learn more?

(Another definition for differs that I've seen is " Varies".)

I've seen this clue in the Evening Standard.
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