Crossword Genius

A grandfather with silly Celtic cross? (10)

Ross

I believe the answer is:

apoplectic

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'cross?' is the definition.
Although both the answer and definition are adjectives, I don't understand how one could define the other.

'a grandfather with silly celtic' is the wordplay.
'a grandfather with' becomes 'apop' (I am not sure about this - if you are sure you should believe this answer much more).
'silly' indicates an anagram.
'celtic' is an anagram of 'lectic'.
'apop'+'lectic'='APOPLECTIC'

Can you help me to learn more?

(Other definitions for apoplectic that I've seen before include "Overcome with anger" , "With red vision" , "Seeing red" , "Outraged" .)

I've seen this clue in the Sydney Morning Herald.
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