A grandfather with silly Celtic cross? (10)
I believe the answer is:
apoplectic
'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" .)