Crossword Genius

Vagrant made Harper switch underwear. It's a fair crop! (6,7)

Ross

I believe the answer is:

bumper harvest

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

'it's a fair crop' is the definition.
I can't tell whether this definition defines the answer.

'vagrant made harper switch underwear' is the wordplay.
'vagrant' becomes 'bum' (I've seen this in another clue).
'made' indicates anagramming the letters.
'switch underwear' becomes 'vest' (I am not sure about the 'switch' bit.).
'harper' with letters rearranged gives 'perhar'.
'bum'+'perhar'+'vest'='BUMPER HARVEST'

Can you help me to learn more?

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