Benefit increase with ever-changing investment? (5,4)

I believe the answer is:

serve well

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'benefit increase' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'with ever-changing investment?' is the wordplay.
'with' means one lot of letters goes inside another.
'changing' indicates anagramming the letters.
'investment?' becomes 'swell' (I can't explain this - if you can you should believe this answer much more).
'ever' is an anagram of 'erve'.
'erve' put within 'swell' is 'SERVE WELL'.

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I've seen this clue in The Guardian.

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