Any year a soup is changed to suit the tax system (3,2,3,4)

I believe the answer is:

pay as you earn

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

'suit the tax system' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'any year a soup is changed' is the wordplay.
'is changed' is an anagram indicator.
'any'+'year'+'a'+'soup'='anyyearasoup'
'anyyearasoup' anagrammed gives 'PAY-AS-YOU-EARN'.

'to' acts as a link.

Can you help me to learn more?

I've seen this clue in The Irish Independent.

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.