A souvenir of achievement (in George the First) is degenerating (10)

I believe the answer is:

atrophying

I'm a little stuck... Click here to teach me more about this clue!

'first is degenerating' is the definition.
I can't judge whether this definition defines the answer.

'a souvenir of achievement in george' is the wordplay.
'souvenir of achievement' becomes 'trophy' (I can't justify this - if you can you should believe this answer much more).
'george' becomes 'g'.
'a'+'trophy'+'in'+'g'='ATROPHYING'

'the' is the link.

Can you help me to learn more?

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

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.